Glossary

Abdomen	The portion of the trunk between the diaphragm and pelvis.
Abduction	The movement of a body part away from the axis or midline of the body; movement of a digit away from the axis of the limb.
ABO system	The most common system of classification for red blood cell antigens. On the basis of antigens on the red blood cell surface, individuals can be type A, type B, type AB, or type O.
Absorption	The transport of molecules across epithelial membranes into the body fluids.
Accessory organs	Organs that assist with the functioning of other organs within a system.
Accommodation	A process whereby the focal length of the eye is changed by automatic adjustment of the curvature of the lens to bring images of objects from various distances into focus on the retina.
Acetabulum	A socket in the lateral surface of the hipbone (os coxa) with which the head of the femur articulates.
Acetone	A ketone body produced as a result of the oxidation of fats.
Acetyl coenzyme A	A coenzyme derivative in the metabolism of glucose and fatty acids that contributes substrates to the Krebs cycle.
Acetylcholine (ACh)	An acetic acid ester of choline-a substance that functions as a neurotransmitter in somatic motor nerve and parasympathetic nerve fibers.
Acetylcholinesterase	An enzyme in the membrane of postsynaptic cells that catalyzes the conversion of ACh into choline and acetic acid. This enzymatic reaction inactivates the neurotransmitter.
Achilles tendon	See tendo calcaneous.
Acid	A substance that releases hydrogen ions when ionized in water.
Acidosis	An abnormal increase in the H+ concentration of the blood that lowers the arterial pH to below 7.35.
Acromegaly	A condition caused by the hypersecretion of growth hormone from the pituitary gland after maturity and characterized by enlargement of the extremities, such as the nose, jaws, fingers, and toes.
Actin	A protein in muscle fibers that together with myosin is responsible for contraction.
Action potential	An all-or-none electrical event in an axon or muscle fiber in which the polarity of the membrane potential is rapidly reversed and re-established.
Active immunity	Immunity involving sensitization, in which antibody production is stimulated by prior exposure to an antigen.
Active transport	The movement of molecules or ions across the cell membranes of epithelial cells by membrane carriers. An expenditure of cellular energy (ATP) is required.
Adduction	The movement of a body part toward the axis or midline of the body; movement of a digit toward the axis of the limb.
Adenohypophysis	The anterior, glandular lobe of the pituitary gland that secretes FSH (follicle-stimulating hormone), LH (luteinizing hormone), ACTH (adrenocorticotropic hormone), TSH (thyroid-stimulating hormone), GH (growth hormone), and prolactin. Secretions of the adenohypophysis are controlled by hormones produced by the hypothalamus.
Adenoids	The tonsils located in the nasopharynx; pharyngeal tonsils.
Adenylate cyclase	An enzyme found in cell membranes that catalyses the conversion of ATP to cyclic AMP and pyrophosphate (PP1). This enzyme is activated by an interaction between a specific hormone and its membrane receptor protein.
ADH	Antidiuretic hormone; a hormone produced by the hypothalamus and released by the posterior pituitary that acts on the kidneys to promote water reabsorption; also known as vasopressin.
ADP	Adenosine diphosphate; a molecule that together with inorganic phosphate is used to make ATP (adenosine triphosphate).
Adrenal cortex	The outer part of the adrenal gland. Derived from embryonic mesoderm, the adrenal cortex secretes corticosteroid hormones (such as aldosterone and hydrocortisone).
Adrenal medulla	The inner part of the adrenal gland. Derived from embryonic postganglionic sympathetic neurons, the adrenal medulla secretes catecholamine hormones-epinephrine and (to a lesser degree) norepinephrine.
Adrenergic	A term used to describe the actions of epinephrine, norepinephrine, or other molecules with similar activity (as in adrenergic receptor and adrenergic stimulation).
Adventitia	The outermost epithelial layer of a visceral organ; also called serosa.
Afferent	Conveying or transmitting to.
Afferent arteriole	A blood vessel within the kidney that supplies blood to the glomerulus.
Afferent neuron	See sensory neuron.
Agglutinate	A clump of cells (usually erythrocytes) formed as a result of specific chemical interaction between surface antigens and antibodies.
Agranular leukocytes	White blood cells (leukocytes) that do not contain cytoplasmic granules; specifically, lymphocytes and monocytes.
Albumin	A water-soluble protein produced in the liver; the major component of the plasma proteins.
Aldosterone	The principal corticosteroid hormone involved in the regulation of electrolyte balance (mineralocorticoid).
Alimentary canal	The tubular portion of the digestive tract. See also gastrointestinal tract (GI tract).
Allantois	An extraembryonic membranous sac involved in the formation of blood cells. It gives rise to the fetal umbilical arteries and vein and also contributes to the formation of the urinary bladder.
Allergens	Antigens that evoke an allergic response rather than a normal immune response.
Allergy	A state of hypersensitivity caused by exposure to allergens. It results in the liberation of histamine and other molecules with histamine like effects.
All-or-none principle	The statement of the fact that muscle fibers of a motor unit contract to their maximum extent when exposed to a stimulus of threshold strength.
Allosteric	A term used with reference to the alteration of an enzyme's activity as a result of its combination with a regulator molecule. Allosteric inhibition by an end product represents negative feedback control of an enzyme's activity.
Alveolar sacs	A cluster of alveoli that share a common chamber or central atrium.
Alveolus	1.An individual air capsule within the lung. The alveoli are the basic functional units of respiration. 2.The socket that secures a tooth(tooth socket).
Amniocentesis	A procedure in which a sample of amniotic fluid is aspirated to examine suspended cells for various genetic diseases.
Amnion	A developmental membrane surrounding the fetus that contains amniotic fluid.
Amphiarthrosis	A slightly movable articulation in a functional classification of joints.
Amphoteric	Having both acidic and basic characteristics; used to denote a molecule that can be positively or negatively charged, depending on the pH of its environment.
Ampulla	A saclike enlargement of a duct or tube.
Ampulla of Vater	See hepatopancreatic ampulla.
Anabolic steroids	Steroids with androgen like stimulatory effects on protein synthesis.
Anabolism	A phase of metabolism involving chemical reactions within cells that result in the production of larger molecules from smaller ones; specifically, the synthesis of protein, glycogen, and fat.
Anaerobic respiration	A form of cell respiration involving the conversion of glucose to lactic acid in which energy is obtained without the use of molecular oxygen.
Anal canal	The terminal tubular portion of the large intestine that opens through the anus of the GI tract.
Anaphylaxis	An unusually severe allergic reaction that can result in cardiovascular shock and death.
Anastomosis	An interconnecting aggregation of blood vessels or nerves that form a network plexus.
Anatomical position	An erect body stance with the eyes directed interior, the arms at the sides, the palms of the hands facing interior, and the fingers pointing straight down.
Anatomy	The branch of science concerned with the structure of the body and the relationship of its organs.
Androgens	Steroids containing 18 carbons that have masculinizing effects; primarily those hormones (such as testosterone) secreted by the testes, although weaker androgens are also secreted by the adrenal cortex.
Anemia	An abnormal reduction in the red blood cell count, hemoglobin concentration, or hematocrit, or any combination of these measurements. This condition is associated with a decreased ability of the blood to carry oxygen.
Angina pectoris	A thoracic pain, often referred to the left pectoral and arm area, caused by myocardial ischemia.
Angiotensin II	An 8-amino-acid polypeptide formed from angiotensin I(a 10-amino-acid precursor), which in turn is formed from cleavage of a protein(angiotensinogen) by the action of renin(an enzyme secreted by the kidneys). Angiotensin II is a powerful vasoconstrictor and a stimulator of aldosterone secretion from the adrenal cortex.
Anions	Ions that are negatively charged, such as chloride, bicarbonate, and phosphate.
Antagonist	A muscle that acts in opposition to another muscle.
Antebrachium	The forearm.
Anterior	Toward the front; the opposite of posterior, or dorsal.
Anterior pituitary	See adenohypophysis.
Anterior root	The anterior projection of the spinal cord, composed of axons of motor neurons.
Antibodies	Immunoglobin proteins secreted by B lymphocytes that have transformed into plasma cells. Antibodies are responsible for humoral immunity. Their synthesis is induced by specific antigens, and they combine with these specific antigens but not with unrelated antigens.
Anticodon	A base triplet provided by three nucleotides within a loop of transfer RNA that is complementary in its base-pairing properties to a triplet (the codon) in mRNA. The matching of codon to anticodon provides the mechanism for translating the genetic code into a specific sequence of amino acids.
Antigen	A molecule that can induce the production of antibodies and react in a specific manner with antibodies.
Antigenic determinant site	The region of an antigen molecule that specifically reacts with particular antibodies. A large antigen molecule may have a number of such sites.
Antiserum	A serum that contains specific antibodies.
Anus	The terminal opening of the GI tract.
Aorta	The major systemic vessel of the arterial system of the body, emerging from the left ventricle.
Aortic arch	The superior left bend of the aorta between the ascending and descending portions.
Apex	The tip or pointed end of a conical structure.
Aphasia	Defects in speech, writing, or in the comprehension of spoken or written language caused by brain damage or disease.
Apneustic center	A collection of nuclei(nerve cell bodies) in the brain stem that participates in the rhythmic control of breathing.
Apocrine gland	A type of sweat gland that functions in evaporative cooling. It may respond during periods of emotional stress.
Aponeurosis	A fibrous or membranous sheet like tendon.
Appendix	A short pouch that attaches to the cecum.
Aqueous humor	The watery fluid that fills the anterior and posterior chambers of the eye.
Arachnoid mater	The weblike middle covering(meninx) of the central nervous system.
Arbor vitae	The branching arrangement of white matter within the cerebellum.
Arm	The portion of the upper extremity from the shoulder to the elbow.
Arrector pili muscle	The smooth muscle attached to a hair follicle that, upon contraction, pulls the hair into a more vertical position, resulting in "goose bumps."
Arteriole	A minute arterial branch.
Arteriosclerosis	Any one of a group of diseases characterized by thickening and hardening of the artery wall and in the narrowing of its lumen.
Arteriovenous anastomoses	Direct connections between arteries and veins that bypass capillary beds.
Artery	A blood vessel that carries blood away from the heart.
Arthrology	The scientific study of the structure and function of joints.
Articular cartilage	A hyaline cartilaginous covering over the articulating surface of the bones of synovial joints.
Articulation	A joint.
Arytenoid cartilages	A pair of small cartilages located on the superior aspect of the larynx.
Ascending colon	The portion of the large intestine between the cecum and the hepatic flexure.
Association neuron	A nerve cell located completely within the central nervous system. It conveys impulses in an arc from sensory to motor neurons; also called interneuron or internuncial neuron.
Astigmatism	Unequal curvature of the refractive surfaces of the eye (cornea and/or lens), so that light entering the eye along certain meridians does not focus on the retina.
Atherosclerosis	A common type of arteriosclerosis found in medium and larger arteries in which raised areas within the tunica intima are formed from smooth muscle cells, cholesterol, and other lipids. These plaques occlude arteries and serve as sites for the formation of thrombi.
Atomic number	The number of protons in the nucleus of an atom.
Atopic dermatitis	An allergic skin reaction to agents such as poison ivy and poison oak; a type of delayed hypersensitivity.
ATP	Adenosine triphosphate; the universal energy donor of the cell.
Atretic	Without an opening. Atretic ovarian follicles are those that fail to ovulate.
Atrioventricular bundle	A group of specialized cardiac fibers that conduct impulses from the atrioventricular node to the ventricular muscles of the heart; also called the bundle of His or AV bundle.
Atrioventricular node	A microscopic aggregation of specialized cardiac fibers located in the interatrial septum of the heart that are a part of the conduction system of the heart; AV node.
Atrioventricular valve	A cardiac valve located between an atrium and a ventricle of the heart; AV valve.
Atrium	Either of the two superior chambers of the heart that receive venous blood.
Atrophy	A gradual wasting away or decrease in the size of a tissue or an organ.
Atropine	An alkaloid drug obtained from a plant of the species Belladonna that acts as an anticholinergic agent. It is used medically to inhibit parasympathetic nerve effects, dilate the pupils of the eye, increase the heart rate, and inhibit intestinal movements.
Auditory	Pertaining to the structures of the ear associated with hearing.
Auditory tube	A narrow canal that connects the middle ear chamber to the pharynx; also called the eustachian canal.
Auricle	1.The fleshy pinna of the ear. 2.An ear-shaped appendage of each atrium of the heart.
Autoantibodies	Antibodies formed in response to, and that react with, molecules that are part of one's own body.
Autonomic nervous system	The sympathetic and parasympathetic portions of the nervous system that function to control the actions of the visceral organs and skin; ANS.
Autosomal chromosomes	The paired chromosomes; those other than the sex chromosomes.
Axilla	The depressed hollow commonly called the armpit.
Axon	The elongated process of a nerve cell that transmits an impulse away from the cell body of a neuron.
Basophil	A granular leukocyte that readily stains with basophilic dye.
B cell lymphocytes	Lymphocytes that can be transformed by antigens into plasma cells that secrete antibodies (and are thus responsible for humoral immunity). The B stands for bursa equivalent.
Belly	The thickest circumference of a skeletal muscle.
Benign	Not malignant.
Bifurcate	Forked; divided into two branches.
Bile	A liver secretion that is stored and concentrated in the gallbladder and released through the common bile duct into the duodenum. It is essential for the absorption of fats.
Bilirubin	Bile pigment derived from the breakdown of the heme portion of hemoglobin.
Bipennate	Denoting muscles that have a fibre architecture coursing obliquely on both sides of a tendon.
Blastula	An early stage of prenatal development between the morula and embryonic period.
Blood	The fluid connective tissue that circulates through the cardiovascular system to transport substances throughout the body.
Blood-brain barrier	A specialized mechanism that inhibits the passage of certain materials from the blood into brain tissue and cerebrospinal fluid.
Bolus	A moistened mass of food that is swallowed from the oral cavity into the pharynx.
Bone	A solid, rigid, ossified connective tissue forming an organ of the skeletal system.
Bony labyrinth	A series of chambers within the petrous part of the temporal bone associated with the vestibular organs and the cochlea. The bony labyrinth contains a fluid called perilymph.
Bowman's capsule	See glomerular capsule.
Brachial plexus	A network of nerve fibers that arise from spinal nerves C5-C8 and T1. Nerves arising from the brachial plexuses supply the upper extremities.
Bradycardia	A slow cardiac rate; fewer than 60 beats per minute.
Bradykinins	Short polypeptides that stimulate vasodilation and other cardiovascular changes.
Brain	The enlarged superior portion of the central nervous system located in the cranial cavity of the skull.
Brain stem	The portion of the brain consisting of the medulla oblongata, pons, and midbrain.
Bronchial tree	The bronchi and their branching bronchioles.
Bronchiole	A small division of a bronchus within the lung.
Bronchus	A branch of the trachea that leads to a lung.
Buccal cavity	The mouth, or oral cavity.
Buffer	A molecule that serves to prevent large changes in pH by either combining with H+ or by releasing H+ into solution.
Bulbourethral glands	A pair of glands that secrete a viscous fluid into the male urethra during sexual excitement; also called Cowper's glands. bundle of His See atrioventricular bundle.
Bursa	A saclike structure filled with synovial fluid. Bursae are located at friction points, as around joints, over which tendons can slide without contacting bone.
Buttocks	The rump or fleshy masses on the posterior aspect of the lower trunk, formed primarily by the gluteal muscles.
Calcitonin	Also called thyrocalcitonin. A polypeptide hormone produced by the parafollicular cells of the thyroid and secreted in response to hypercalcemia. It acts to lower blood calcium and phosphate concentrations and may serve as an antagonist of parathyroid hormones.
Calmodulin	A receptor protein for Ca++ located within the cytoplasm of target cells. It appears to mediate the effects of this ion on cellular activities.
Calorie	A unit of heat equal to the amount needed to raise the temperature of one gram of water by 1 C∞.
Calyx	A cup-shaped portion of the renal pelvis that encircles a renal papilla.
Canaliculus	A microscopic channel in bone tissue that connects lacunae.
Canal of Schlemm	See scleral venous sinus.
Cancer	A tumour characterized by abnormally rapid cell division and the loss of specialized tissue characteristics. This term usually refers to malignant tumours.
Capacitation	The process whereby spermatozoa gain the ability to fertilize ova. Sperm that have not have been capacitated in the female reproductive tract cannot fertilize ova.
Capillary	A microscopic blood vessel that connects an arteriole and a venule; the functional unit of the circulatory system.
Carbonic anhydrase	An enzyme that catalyses the formation or breakdown of carbonic acid. When carbon dioxide concentrations are relatively high, this enzyme catalyses the formation of carbonic acid from CO2 and H2O. When carbon dioxide concentrations are low, the breakdown of carbonic acid to CO2 and H2O is catalysed. These reactions aid the transport of carbon dioxide from tissues to alveolar air.
Cardiac muscle	Muscle of the heart, consisting of striated muscle cells. These cells are interconnected into a mass called the myocardium.
Cardiac output	The volume of blood pumped per minute by either the right or left ventricle.
Cardiogenic shock	Shock that results from low cardiac output in heart disease.
Carotid sinus	An expanded portion of the internal carotid artery located immediately above the point of branching from the external carotid artery. The carotid sinus contains baroreceptors that monitor blood pressure.
Carpus	The proximal portion of the hand that contains the eight carpal bones.
Carrier-mediated transport	The transport of molecules or ions across a cell membrane by means of specific protein carriers. It includes both facilitated diffusion and active transport.
Cartilage	A type of connective tissue with a solid elastic matrix.
Cartilaginous joint	A joint that lacks a joint cavity, permitting little movement between the bones held together by cartilage.
Cast	An accumulation of proteins molded from the kidney tubules that appears in urine sediment
Catabolism	The metabolic breakdown of complex molecules into simpler ones, often resulting in a release of energy.
Catecholamines	A group of molecules including epinephrine, norepinephrine, L-dopa, and related molecules with effects similar to those produced by activation of the sympathetic nervous system.
Cations	Positively charged ions, such as sodium, potassium, calcium, and magnesium.
Cauda equina	The lower end of the spinal cord where the roots of spinal nerves have a tail-like appearance.
Cecum	The pouchlike portion of the large intestine to which the ileum of the small intestine is attached.
Cell	The structural and functional unit of an organism; the smallest structure capable of performing all the functions necessary for life.
Cell-mediated immunity	Immunological defence provided by T cell lymphocytes that come within close proximity of their victim cells(as opposed to humoral immunity provided by the secretion of antibodies by plasma cells).
Cellular respiration	The energy-releasing metabolic pathways in a cell that oxidize organic molecules such as glucose and fatty acids.
Cementum	Bonelike material that binds the root of a tooth to the periodontal membrane of the bony socket.
Central canal	An elongated longitudinal channel in the centre of an osteon in bone tissue that contains branches of the nutrient vessels and a nerve; also called a haversian canal.
Central nervous system	Part of the nervous system consisting of the brain and the spinal cord; CNS.
Centrioles	Cell organelles that form the spindle apparatus during cell division.
Centromere	The central region of a chromosome to which the chromosomal arms are attached.
Centrosome	A dense body near the nucleus of a cell that contains a pair of centrioles.
Cerebellar peduncle	An aggregation of nerve fibers connecting the cerebellum with the brain stem.
Cerebellum	The portion of the brain concerned with the coordination of skeletal muscle contraction. Part of the metencephalon, it consists of two hemispheres and a central vermis.
Cerebral arterial circle	An arterial vessel that encircles the pituitary gland. It provides alternate routes for blood to reach the brain should a carotid or vertebral artery become occluded; also called the circle of Willis.
Cerebral peduncles	A paired bundle of nerve fibers along the inferior surface of the midbrain that conduct impulses between the pons and the cerebral hemispheres.
Cerebrospinal fluid	A fluid produced by the choroid plexus of the ventricles of the brain. It fills the ventricles and surrounds the central nervous system in association with the meninges.
Cerebrum	The largest portion of the brain, composed of the right and left hemispheres.
Ceruminous gland	(sue-roo´mu1-nus) A specialised integumentary gland that secretes cerumen, or earwax, into the external auditory canal.
Cervical	Pertaining to the neck or a necklike portion of an organ.
Cervical ganglion	A cluster of postganglionic sympathetic nerve cell bodies located in the neck, near the cervical vertebrae.
Cervical plexus	A network of spinal nerves formed by the anterior branches of the first four cervical nerves.
Cervix	1.The narrow necklike portion of an organ. 2.The inferior end of the uterus that adjoins the vagina(cervix of the uterus).
Chemoreceptor	A neuroreceptor that is stimulated by the presence of chemical molecules.
Chemotaxis	The movement of an organism or a cell, such as a leukocyte, toward a chemical stimulus.
Cheyne-Stokes respiration	Breathing characterized by rhythmic waxing and waning of the depth of respiration, with regularly occurring periods of apnea (failure to breathe).
Chiasma	A crossing of nerve tracts from one side of the CNS to the other; also called a chiasm.
Choane	The two posterior openings from the nasal cavity into the nasal pharynx; also called the internal nares.
Cholesterol	A 27-carbon steroid that serves as the precursor of steroid hormones.
Cholinergic	Denoting nerve endings that liberate acetylcholine as a neurotransmitter, such as those of the parasympathetic system.
Chondrocranium	The portion of the skull that supports the brain. It is derived from endochondral bone.
Chondrocytes	Cartilage-forming cells.
Chordae tendineae	Cordlike tendinous bands that connect papillary muscles to the leaflets of the atrioventricular valves within the ventricles of the heart.
Chorea	The occurrence of a wide variety of rapid, complex, jerky movements that appear to be well coordinated but that are performed involuntarily.
Chorion	An extraembryonic membrane that participates in the formation of the placenta.
Choroid	The vascular, pigmented middle layer of the wall of the eye.
Choroid plexus	A mass of vascular capillaries from which cerebrospinal fluid is secreted into the ventricles of the brain.
Chromatids	Duplicated chromosomes, joined together at the centromere, that separate during cell division.
Chromatin	Threadlike structures in the cell nucleus consisting primarily of DNA and protein. They represent the extended form of chromosomes during interphase.
Chromatophilic substances	Clumps of rough endoplasmic reticulum in the cell bodies of neurons; also called Nissl bodies.
Chromosomes	Structures in the nucleus that contain the genes for genetic expression.
Chyme	The mass of partially digested food that passes from the pylorus of the stomach into the duodenum of the small intestine.
Cilia	Microscopic hairlike processes that move in a wavelike manner on the exposed surfaces of certain epithelial cells.
Ciliary body	A portion of the choroid layer of the eye that secretes aqueous humour. It contains the ciliary muscle.
Circadian rhythms	Physiological changes that repeat at about 24-hour intervals. These are often synchronized with changes in the external environment, such as the day-night cycles.
Circle of Willis	See cerebral arterial circle.
Circumduction	A movement of a body part that outlines a cone, such that the distal end moves in a circle while the proximal portion remains relatively stable.
Cirrhosis	Liver disease characterized by loss of normal microscopic structure, which is replaced by fibrosis and nodular regeneration.
Clitoris	A small, erectile structure in the vulva of the female, homologous to the glans penis in the male.
Clone	1.A group of cells derived from a single parent cell by mitotic cell division; since reproduction is asexual, the descendants of the parent cell are genetically identical. 2. A term used to refer to cells as separate individuals(as in white blood cells) rather than as part of a growing organ.
CNS	See central nervous system.
Coccygeal	Pertaining to the region of the coccyx; the caudal termination of the vertebral column.
Cochlea	The organ of hearing in the inner ear where nerve impulses are generated in response to sound waves.
Cochlear window	See round window.
Codon	The sequence of three nucleotide bases in mRNA that specifies a given amino acid and determines the position of that amino acid in a polypeptide chain through complementary base pairing with an anticodon in tRNA.
Coelom	The abdominal cavity.
Coenzyme	An organic molecule, usually derived from a water-soluble vitamin, that combines with and activates specific enzyme proteins.
Cofactor	A substance needed for the catalytic action of an enzyme; generally used in reference to inorganic ions such as Ca++ and Mg++.
Collateral	A small branch of a blood vessel or nerve fibre.
Colloid osmotic pressure	Osmotic pressure exerted by plasma proteins that are present as a colloidal suspension; also called oncotic pressure.
Colon	The first portion of the large intestine.
Common bile duct	A tube formed by the union of the hepatic duct and cystic duct that transports bile to the duodenum.
Compact bone	Tightly packed bone that is superficial to spongy bone and covered by the periosteum; also called dense bone.
Compliance	A measure of the ease with which a structure such as the lung expands under pressure; a measure of the change in volume as a function of pressure changes.
Conduction myofibers	Specialized large-diameter cardiac muscle fibers that conduct electrical impulses from the AV bundle into the ventricular walls; also called Purkinje fibers.
Condyle	A rounded process at the end of a long bone that forms an articulation.
Cone	A colour receptor cell in the retina of the eye.
Congenital	Present at the time of birth.
Congestive heart failure	The inability of the heart to deliver an adequate blood flow as a result of heart disease or hypertension. This condition is associated with breathlessness, salt and water retention, and edema.
Conjunctiva	The thin membrane covering the anterior surface of the eyeball and lining the eyelids.
Conjunctivitis	Inflammation of the conjunctiva of the eye, which is sometimes called "pink eye."
Connective tissue	One of the four basic tissue types within the body. It is a binding and supportive tissue with abundant matrix.
Conn's syndrome	Primary hyperaldosteronism; excessive secretion of aldosterone produces electrolyte imbalances.
Contralateral	Taking place or originating in a corresponding part on the opposite side of the body.
Conus medullaris	The inferior, tapering portion of the spinal cord.
Convolution	An elevation on the surface of a structure and an infolding of the tissue upon itself.
Cornea	The transparent, convex, anterior portion of the outer layer of the eyeball.
Coronal plane	A plane that divides the body into anterior and posterior portions; also called a frontal plane.
Coronary circulation	The arterial and venous blood circulation to the wall of the heart.
Coronary sinus	A large venous channel on the posterior surface of the heart into which the cardiac veins drain.
Corpora quadrigemina	Four superior lobes of the midbrain concerned with visual and auditory functions.
Corpus callosum	A large tract of white matter within the brain that connects the right and left cerebral hemispheres.
Corpuscle of touch	A touch sensory receptor found in the papillary layer of the dermis of the skin; also called Meissner's corpuscle.
Cortex	1.The outer layer of an internal organ or body structure, as of the kidney or adrenal gland. 2. The convoluted layer of grey matter that covers the surface of each cerebral hemisphere.
Corticosteroids	Steroid hormones of the adrenal cortex, consisting of glucocorticoid (such as hydrocortisone) and mineralocorticoids (such as aldosterone).
Costal cartilage	The cartilage that connects the ribs to the sternum.
Cranial	Pertaining to the cranium.
Cranial nerves	One of 12 pairs of nerves that arise from the brain.
Cranium	The bones of the skull that enclose or support the brain and the organs of sight, hearing, and balance.
Creatine phosphate	An organic phosphate molecule in muscle cells that serves as a source of high-energy phosphate for the synthesis of ATP; also called phosphocreatine.
Crenation	A notched or scalloped appearance of the red blood cell membrane caused by the osmotic loss of water from these cells.
Crest	A thickened ridge of bone for the attachment of muscle.
Cretinism	A condition caused by insufficient thyroid secretion during prenatal development or the years of early childhood. It results in stunted growth and inadequate mental development.
Cricoid cartilage	A ring-shaped cartilage that forms the inferior portion of the larynx.
Crista	A crest, such as the crista galli that extends superiorly from the cribriform plate of the ethmoid bone.
Cryptorchidism	A developmental defect in which one or both testes fail to descend into the scrotum and, instead, remain in the body cavity.
Cubital	Pertaining to the antebrachium. The cubital fossa is the anterior aspect of the elbow joint.
Curare	A chemical derived from plant sources that causes flaccid paralysis by blocking ACh receptor proteins in muscle cell membranes.
Cushing's syndrome	Symptoms caused by the hypersecretion of adrenal steroid hormones as a result of tumours of the adrenal cortex or ACTH-secreting tumours of the anterior pituitary.
Cyanosis	A bluish discoloration of the skin or mucous membranes due to excessive concentration of deoxyhemoglobin; indicates inadequate oxygen concentration in the blood.
Cystic duct	The tube that transports bile from the gallbladder to the common bile duct.
Cytochrome	A pigment in mitochondria that transports electrons in the process of aerobic respiration.
Cytokinesis	The division of the cytoplasm that occurs in mitosis and meiosis, when a parent cell divides to produce two daughter cells.
Cytology	The science dealing with the study of cells.
Cytoplasm	In a cell, the protoplasm located outside of the nucleus.
Cytoskeleton	A latticework of structural proteins in the cytoplasm arranged in the form of microfilaments and microtubules.
Deciduous	Pertaining to something shed or cast off in a particular sequence. Deciduous teeth are shed and replaced by permanent teeth during development.
Decussation	A crossing of nerve fibers from one side of the CNS to the other.
Deglutition	The act of swallowing.
Delayed hypersensitivity	An allergic response in which the onset of symptoms may not occur until 2 or 3 days after exposure to an antigen. Produced by T cells, it is a type of cell-mediated immunity.
Denaturation	Irreversible changes in the tertiary structure of proteins caused by heat or drastic pH changes.
Dendrite	A nerve cell process that transmits impulses toward a neuron cell body.
Dentin	The main substance of a tooth, covered by enamel over the crown of the tooth and by cementum on the root.
Dentition	The number, arrangement, and shape of teeth.
Depolarization	The loss of membrane polarity in which the inside of the cell membrane becomes less negative in comparison to the outside of the membrane. The term is also used to indicate the reversal of membrane polarity that occurs during the production of action potentials in nerve and muscle cells.
Dermal papilla	A projection of the dermis into the epidermis.
Dermis	The second, or deep, layer of skin beneath the epidermis.
Descending colon	The segment of the large intestine that descends on the left side from the level of the spleen to the level of the left iliac crest.
Diabetes insipidus	A condition in which inadequate amounts of antidiuretic hormone(ADH) are secreted by the posterior pituitary. It results in the inadequate reabsorption of water by the kidney tubules and, thus, in the excretion of a large volume of dilute urine.
Diabetes mellitus	The appearance of glucose in the urine due to the presence of high plasma glucose concentrations, even in the fasting state. This disease is caused by either lack of sufficient insulin secretion or inadequate responsiveness of the target tissues to the effects of insulin.
Diapedesis	The migration of white blood cells through the endothelial walls of blood capillaries into the surrounding connective tissues.
Diaphragm	A sheet like dome of muscle and connective tissue that separates the thoracic and abdominal cavities.
Diaphysis	The shaft of a long bone.
Diarthrosis	A type of functionally classified joint in which the articulating bones are freely movable; also called a synovial joint.
Diastole	The sequence of the cardiac cycle during which a heart chamber wall is relaxed.
Diencephalon	A major region of the brain that includes the third ventricle, thalamus, hypothalamus, and pituitary gland.
Diffusion	The net movement of molecules or ions from regions of higher to regions of lower concentration.
Digestion	The process by which larger molecules of food substance are broken down mechanically and chemically into smaller molecules that can be absorbed.
Diploe	The spongy layer of bone positioned between the inner and outer layers of compact bone.
Diploid	Denoting cells having two of each chromosome or twice the number of chromosomes that are present in sperm or ova.
Disaccharide	Any of a class of double sugars; carbohydrates that yield two simple sugars, or monosaccharides, upon hydrolysis.
Distal	Away from the midline or origin; the opposite of proximal.
Diuretic	An agent that promotes the excretion of urine, thereby lowering blood volume and pressure.
DNA	Deoxyribonucleic acid; composed of nucleotide bases and deoxyribose sugar. It is found in all living cells and contains the genetic code.
Dopamine	A type of neurotransmitter in the central nervous system; also is the precursor of norepinephrine, another neurotransmitter molecule.
Dorsal	Pertaining to the back or posterior portion of a body part; the opposite of ventral; also called posterior.
Dorsal root ganglion	See posterior root ganglion.
Dorsiflexion	Movement at the ankle as the dorsum of the foot is elevated.
Ductus arteriosus	The blood vessel that connects the pulmonary trunk and the aorta in a fetus.
Ductus deferens	A tube that carries spermatozoa from the epididymis to the ejaculatory duct; also called the vas deferens or seminal duct.
Ductus venosus	A fetal blood vessel that connects the umbilical vein and the inferior vena cava.
Duodenum	The first portion of the small intestine that leads from the pylorus of the stomach to the jejunum.
Dura mater	The outermost meninx.
Dwarfism	A condition in which a person is undersized due to inadequate secretion of growth hormone.
Dyspnea	Subjective difficulty in breathing.
Eccrine gland	A sweat gland that functions in thermoregulation.
ECG	See electrocardiogram.
Ectoderm	The outermost of the three primary germ layers of an embryo.
Ectopic focus	An area of the heart other than the SA node that assumes pacemaker activity.
Ectopic pregnancy	Embryonic development that occurs anywhere other than in the uterus (as in the uterine tubes or body cavity).
Effector	An organ, such as a gland or muscle, that responds to a motor stimulation.
Efferent	Conveying away from the centre of an organ or structure.
Efferent arteriole	An arteriole of the renal vascular system that conducts blood away from the glomerulus of a nephron.
Efferent ductules	A series of coiled tubules through which spermatozoa are transported from the rete testis to the epididymis.
Efferent neuron	See motor neuron.
Ejaculation	The discharge of semen from the male urethra that accompanies orgasm.
Ejaculatory duct	A tube that transports spermatozoa from the ductus deferens to the prostatic urethra.
Elastic fibers	Protein strands that are found in certain connective tissue that have contractile properties.
Elbow	The synovial joint between the brachium and the antebrachium.
Electrocardiogram	A recording of the electrical activity that accompanies the cardiac cycle; ECG or EKG.
Electroencephalogram	A recording of the brain-wave patterns or electrical impulses of the brain from electrodes placed on the scalp; EEG.
Electrolytes	Ions and molecules that are able to ionize and thus carry an electric current. The most common electrolytes in the plasma are Na+, HCO3-, and K+.
Electromyogram	A recording of the electrical impulses or activity of skeletal muscles using surface electrodes; EMG.
Electrophoresis	A biochemical technique in which different molecules can be separated and identified by their rate of movement in an electric field.
Elephantiasis	A disease caused by infection with a nematode worm in which the larvae block lymphatic drainage and produce edema; the lower areas of the body can become enormously swollen as a result.
Embryology	The study of prenatal development from conception through the eighth week in utero.
EMG	See electromyogram.
Emphysema	A lung disease in which the alveoli are destroyed and the remaining alveoli become larger. It results in decreased vital capacity and increased airway resistance.
Emulsification	The process of producing an emulsion or fine suspension; in the small intestine, fat globules are emulsified by the detergent action of bile.
Enamel	The outer dense substance covering the crown of a tooth.
Endergonic	Denoting a chemical reaction that requires the input of energy from an external source in order to proceed.
Endocardium	The endothelial lining of the heart chambers and valves.
Endochondral bone	Denoting bones that develop as hyaline cartilage models first and that are then ossified.
Endocrine gland	A ductless, hormone-producing gland that is part of the endocrine system.
Endocytosis	A general term for the cellular uptake of particles that are too large to cross the cell membrane. See also phagocytosis and pinocytosis.
Endoderm	The innermost of the three primary germ layers of an embryo.
Endogenous	Denoting a product or process arising from within the body (as opposed to exogenous products or influences from external sources).
Endolymph	A fluid within the membranous labyrinth and cochlear duct of the inner ear that aids in the conduction of vibrations involved in hearing and the maintenance of equilibrium.
Endometrium	The inner lining of the uterus.
Endomysium	The connective tissue sheath that surrounds each skeletal muscle fibre, separating the muscle cells from one another.
Endoneurium	The connective tissue sheath that surrounds each nerve fibers, separating the nerve fibers one from another within a nerve.
Endoplasmic reticulum	A cytoplasmic organelle composed of a network of canals running through the cytoplasm of a cell.
Endorphins	A group of endogenous opiate molecules that may act as a natural analgesic.
Endothelium	The layer of epithelial tissue that forms the thin inner lining of blood vessels and heart chambers.
Endotoxin	A toxin found within certain types of bacteria that is able to stimulate the release of endogenous pyrogen and produce a fever.
Enkephalins	Short polypeptides, containing five amino acids, that have analgesic effects and that may function as neurotransmitters in the brain. The two known enkephalins (which differ in only one amino acid) are endorphins.
Enteric	The term referring to the small intestine.
Entropy	The energy of a system that is not available to perform work. A measure of the degree of disorder in a system, entropy increases whenever energy is transformed.
Enzyme	A protein catalyst that increases the rate of specific chemical reactions.
Eosinophil	A type of white blood cell characterized by the presence of cytoplasmic granules that become stained by acidic eosin dye. Eosinophils normally constitute about 2% to 4% of the white blood cells.
Epicardium	A thin, outer layer of the heart; also called the visceral pericardium.
Epicondyle	A projection of bone above a condyle.
Epidermis	The outermost layer of the skin, composed of several stratified squamous epithelial layers.
Epididymis	A highly coiled tube located along the posterior border of the testis. It stores spermatozoa and transports them from the seminiferous tubules of the testis to the ductus deferens.
Epidural space	A space between the spinal dura mater and the bone of the vertebral canal.
Epiglottis	A leaf like structure positioned on top of the larynx. It covers the glottis during swallowing.
Epimysium	A fibrous outer sheath of connective tissue surrounding a skeletal muscle.
Epinephrine	A hormone secreted from the adrenal medulla resulting in actions similar to those resulting from sympathetic nervous system stimulation; also called adrenaline.
Epineurium	A fibrous outer sheath of connective tissue surrounding a nerve.
Epiphyseal plate	A hyaline cartilaginous layer located between the epiphysis and diaphysis of a long bone. It functions as a longitudinal growing region.
Epiphysis	The end segment of a long bone, separated from the diaphysis early in life by an epiphyseal plate but later becoming part of the larger bone.
Episiotomy	An incision of the perineum at the end of the second stage of labour to facilitate delivery and to avoid tearing the perineum.
Epithelial tissue	One of the four basic tissue types; the type of tissue that covers or lines all exposed body surfaces.
Eponychium	The thin layer of stratum corneum of the epidermis of the skin that overlaps and protects the lunula of the nail.
EPSP	Excitatory postsynaptic potential; a graded depolarization of a postsynaptic membrane in response to stimulation by a neurotransmitter chemical. EPSPs can be summated but can be transmitted only over short distances. They can stimulate the production of action potentials when a threshold level of depolarization has been attained.
Erythroblastosis fetalis	Hemolytic anemia in an Rh positive newborn caused by maternal antibodies against the Rh factor that have crossed the placenta.
Erythrocyte	A red blood cell.
Esophagus	A tubular portion of the GI tract that leads from the pharynx to the stomach as it passes through the thoracic cavity.
Essential amino acids	Those eight amino acids in adults or nine amino acids in children that cannot be made by the human body; therefore, they must be obtained in the diet.
Estrogens	Any of several female sex hormones secreted from the ovarian (graafian) follicle.
Estrus cycle	Cyclic changes in the structure and function of the ovaries and female reproductive tract of mammals other than humans, accompanied by periods of "heat" (estrus) or sexual receptivity. Estrus is the equivalent of the human menstrual cycle but differs from the human menstrual cycle in that the endometrium is not shed with accompanying bleeding.
Etiology	The study of cause, especially of disease, including the origin and what pathogens, if any, are involved.
Eversion	A movement of the foot in which the sole is turned outward.
Exergonic	Denoting chemical reactions that liberate energy.
Exocrine gland	A gland that secretes its product to an epithelial surface, directly or through ducts.
Exocytosis	The process of cellular secretion in which the secretory products are contained within a membrane-enclosed vesicle. The vesicle fuses with the cell membrane so that the lumen of the vesicle is open to the extracellular environment.
Expiration	The process of expelling air from the lungs through breathing out; also called exhalation.
Extension	A movement that increases the angle between parts of a joint.
Extensor	A muscle that, upon contraction, increases the angle of a joint.
External acoustic meatus	An opening through the temporal bone that connects with the tympanum and the middle-ear chamber and through which sound vibrations pass; also called the external auditory meatus.
Exteroceptors	Sensory receptors that are sensitive to changes in the external environment (as opposed to interoceptors).
Extraocular muscles	The muscles that insert into the sclera of the eye and that act to change the position of the eye in its orbit (as opposed to the intraocular muscles, such as those of the iris and ciliary body within the eye).
Extrinsic	Pertaining to an outside or external origin.
Face	1. The anterior aspect of the head not supporting or covering the brain. 2. The exposed surface of a structure.
Facet	A small, smooth surface of a bone where articulation occurs.
Facilitated diffusion	The carrier-mediated transport of molecules through the cell membrane along the direction of their concentration gradients. It does not require the expenditure of metabolic energy.
FAD	Flavin adenine dinucleotide; a coenzyme derived from riboflavin that participates in electron transport within the mitochondria.
Falciform ligament	An extension of parietal peritoneum that separates the right and left lobes of the liver.
Fallopian tube	See uterine tube.
False vocal cords	The supporting folds of tissue for the true vocal cords within the larynx.
Falx cerebelli	A fold of the dura mater anchored to the occipital bone. It projects inward between the cerebellar hemispheres.
Falx cerebri	A fold of dura mater anchored to the crista galli of the ethmoid bone. It extends between the right and left cerebral hemispheres.
Fascia	A tough sheet of fibrous tissue binding the skin to underlying muscles or supporting and separating muscles.
Fasciculus	A small bundle of muscle or nerve fibers.
Fauces	The passageway between the mouth and the pharynx.
Feces	Material expelled from the GI tract during defecation, composed of undigested food residue, bacteria, and secretions; also called stool.
Fertilization	The fusion of an ovum and spermatozoon.
Fetus	A prenatal human after 8 weeks of development.
Fibrillation	A condition of cardiac muscle characterized electrically by random and continuously changing patterns of electrical activity and resulting in the inability of the myocardium to contract as a unit and pump blood. It can be fatal if it occurs in the ventricles.
Fibrin	The insoluble protein formed from fibrinogen by the enzymatic action of thrombin during the process of blood clot formation.
Fibrinogen	A soluble plasma protein that serves as the precursor of fibrin; also called factor I.
Fibroblast	An elongated connective tissue cell with cytoplasmic extensions that is capable of forming collagenous fibers or elastic fibers.
Fibrous joint	A type of articulation bound by fibrous connective tissue that allows little or no movement (e.g., a syndesmosis).
Filiform papillae	Numerous small projections over the entire surface of the tongue in which taste buds are absent.
Filum terminale	A fibrous, threadlike continuation of the pia mater, extending inferiorly from the terminal end of the spinal cord to the coccyx.
Fimbriae	Fringelike extensions from the borders of the open end of the uterine tube.
Fissure	A groove or narrow cleft that separates two parts, such as the cerebral hemispheres of the brain.
Flagellum	A whip like structure that provides motility for sperm.
Flare-and-wheal reaction	A cutaneous reaction to skin injury or the administration of antigens, produced by release of histamine and related molecules and characterized by local edema and a red flare.
Flavoprotein	A conjugated protein containing a flavin pigment that is involved in electron transport within the mitochondria.
Flexion	A movement that decreases the angle between parts of a joint.
Flexor	A muscle that decreases the angle of a joint when it contracts.
Fontanel	A membranous-covered region on the skull of a fetus or baby where ossification has not yet occurred; commonly called a soft spot.
Foot	The terminal portion of the lower extremity, consisting of the tarsal bones, metatarsal bones, and phalanges.
Foramen	An opening in an anatomical structure, usually in a bone, for the passage of a blood vessel or a nerve.
Foramen ovale	An opening through the interatrial septum of the fetal heart.
Forearm	The portion of the upper extremity between the elbow and the wrist; also called the antebrachium.
Fornix	1. A recess around the cervix of the uterus where it protrudes into the vagina. 2. A tract within the brain connecting the hippocampus with the mamillary bodies.
Fossa	A depressed area, usually on a bone.
Fourth ventricle	A cavity within the brain, between the cerebellum and the medulla oblongata and the pons, containing cerebrospinal fluid.
Fovea centralis	A depression on the macula lutea of the eye, where only cones are located; the area of keenest vision.
Frenulum	A membranous structure that serves to anchor and limit the movement of a body part.
Frontal	1. Pertaining to the region of the forehead. 2. A plane through the body, dividing the body into anterior and posterior portions; also called the coronal plane.
FSH	Follicle-stimulating hormone; one of the two gonadotropic hormones secreted from the anterior pituitary. In females, FSH stimulates the development of the ovarian follicles; in males, it stimulates the production of sperm in the seminiferous tubules.
Fungiform papillae	Flattened, mushroom-shaped projections interspersed over the surface of the tongue in which taste buds are present.
GABA	Gamma-aminobutyric acid; believed to function as an inhibitory neurotransmitter in the central nervous system.
Gallbladder	A pouch like organ attached to the underside of the liver in which bile secreted by the liver is stored and concentrated.
Gamete	A haploid sex cell; either an egg cell or a sperm cell.
Ganglion	An aggregation of nerve cell bodies occurring outside the central nervous system.
Gastric intrinsic factor	A glycoprotein secreted by the stomach that is needed for the absorption of vitamin B12.
Gastrin	A hormone secreted by the stomach that stimulates the gastric secretion of hydrochloric acid and pepsin.
Gastrointestinal tract	The portion of the digestive tract that includes the stomach and the small and large intestines.
Gates	Structures composed of one or more protein molecules that regulate the passage of ions through channels within the cell membrane. Gates may be chemically regulated (by neurotransmitters) or voltage regulated (in which case they open in response to a threshold level of depolarization).
Genetic recombination	The formation of new combinations of genes, as by crossing-over between homologous chromosomes.
Genetic transcription	The process by which RNA is produced with a sequence of nucleotide bases that is complementary to a region of DNA.
Genetic translation	The process by which proteins are produced with amino acid sequences specified by the sequence of codons in messenger RNA.
Gigantism	Abnormal body growth as a result of the excessive secretion of growth hormone.
Gingiva	The fleshy covering over the mandible and maxilla through which the teeth protrude within the mouth; also called the gum.
Gland	An organ that produces a specific substance or secretion.
Glans penis	The enlarged, sensitive, distal end of the penis.
Gliding joint	A type of synovial joint in which the articular surfaces are flat, permitting only side-to-side and back-and-forth movements.
Glomerular capsule	The double-walled proximal portion of a renal tubule that encloses the glomerulus of a nephron; also called Bowman's capsule.
Glomerular filtration rate	The volume of filtrate produced per minute by both kidneys.
Glomerular ultrafiltrate	Fluid filtered through the glomerular capillaries into the glomerular capsule of the kidney tubules.
Glomerulonephritis	Inflammation of the renal glomeruli, associated with fluid retention, edema, hypertension, and the appearance of protein in the urine.
Glomerulus	A coiled tuft of capillaries surrounded by the glomerular capsule that filtrates urine from the blood.
Glottis	A slit like opening into the larynx, positioned between the true vocal cords.
Glucagon	A polypeptide hormone secreted by the alpha cells of the pancreatic islets. It acts primarily on the liver to promote glycogenolysis and raise blood glucose levels.
Glucocorticoids	Steroid hormones secreted by the adrenal cortex (corticosteroids). They affect the metabolism of glucose, protein, and fat and also have anti-inflammatory and immunosuppressive effects. The major glucocorticoid in humans is hydrocortisone (cortisol).
Gluconeogenesis	The formation of glucose from noncarbohydrate molecules, such as amino acids and lactic acid.
Glycerol	A 3-carbon alcohol that serves as a building block of fats.
Glycogen	A polysaccharide of glucose-also called animal starch-produced primarily in the liver and skeletal muscles. Similar to plant starch in composition, glycogen contains more highly branched chains of glucose subunits than does plant starch.
Glycogenesis	The formation of glycogen from glucose.
Glycogenolysis	The hydrolysis of glycogen to glucose 1-phosphate, which can be converted to glucose 6-phosphate, which then may be oxidized via glycolysis or (in the liver) converted to free glucose.
Glycolysis	The metabolic pathway that converts glucose to pyruvic acid; the final products are two molecules of pyruvic acid and two molecules of reduced NAD, with a net gain of two ATP molecules. In anaerobic respiration, the reduced NAD is oxidized by the conversion of pyruvic acid to lactic acid. In aerobic respiration, pyruvic acid enters the Krebs cycle in mitochondria and reduced NAD is ultimately oxidized to yield water.
Glycosuria	The excretion of an abnormal amount of glucose in the urine (urine normally only contains trace amounts of glucose).
Goblet cell	A unicellular mucus-secreting gland that is associated with columnar epithelia; also called a mucous cell.
Golgi apparatus	A network of stacked, flattened membranous sacs within the cytoplasm of cells. Its major function is to concentrate and package proteins for secretion from the cell.
Golgi tendon organ	A sensory receptor found near the junction of tendons and muscles.
Gonad	A reproductive organ, testis or ovary, that produces gametes and sex hormones.
Gonadotropic hormones	Hormones of the anterior pituitary that stimulate gonadal function-the formation of gametes and secretion of sex steroids. The two gonadotropins are FSH (follicle-stimulating hormone) and LH (luteinizing hormone), which are essentially the same in males and females.
Graafian follicle	A mature ovarian follicle, containing a single fluid-filled cavity, with the ovum located toward one side of the follicle and perched on top of a hill of granulosa cells.
Granular leukocytes	Leukocytes with granules in the cytoplasm; on the basis of the staining properties of the granules, these cells are classified as neutrophils, eosinophils, or basophils.
Graves' disease	A hyperthyroid condition believed to be caused by excessive stimulation of the thyroid gland by autoantibodies; it is associated with exophthalmos (bulging eyes), high pulse rate, high metabolic rate, and other symptoms of hyperthyroidism.
Grey matter	The region of the central nervous system composed of nonmyelinated nerve tissue.
Greater omentum	A double-layered peritoneal membrane that originates on the greater curvature of the stomach. It hangs inferiorly like an apron over the contents of the abdominal cavity.
Gross anatomy	The branch of anatomy concerned with structures of the body that can be studied without a microscope.
Growth hormone	A hormone secreted by the anterior pituitary that stimulates growth of the skeleton and soft tissues during the growing years and that influences the metabolism of protein, carbohydrate, and fat throughout life.
Gustatory	Pertaining to the sense of taste.
Gut	The GI tract or a portion thereof; generally used in reference to the embryonic digestive tube, consisting of the foregut, midgut, and hindgut.
Gyrus	A convoluted elevation or ridge.
Hair	A threadlike appendage of the epidermis consisting of keratinized dead cells that have been pushed up from a dividing basal layer.
Hair cells	Specialised receptor nerve endings for detecting sensations, such as in the spiral organ (organ of Corti).
Hair follicle	A tubular depression in the dermis of the skin in which a hair develops.
Hand	The terminal portion of the upper extremity, containing the carpal bones, metacarpal bones, and phalanges.
Haploid	A cell that has one of each chromosome type and therefore half the number of chromosomes present in most other body cells; only the gametes (sperm and ova) are haploid.
Haptens	Small molecules that are not antigenic by themselves, but which-when combined with proteins-become antigenic and thus capable of stimulating the production of specific antibodies.
Hard palate	The bony partition between the oral and nasal cavities, formed by the maxillae and palatine bones and lined by mucous membrane.
Haustra	Sacculations or pouches of the colon.
Haversian canal	See central canal.
Haversian system	See osteon.
Hay fever	A seasonal type of allergic rhinitis caused by pollen; it is characterized by itching and tearing of the eyes, swelling of the nasal mucosa, attacks of sneezing, and often by asthma.
Head	The uppermost portion of a human that contains the brain and major sense organs.
Heart	A four-chambered, muscular pumping organ positioned in the thoracic cavity, slightly to the left of midline.
Heart murmur	An auscultatory sound of cardiac or vascular origin, usually caused by an abnormal flow of blood in the heart as a result of structural defects of the valves or septum.
Helper T cells	A subpopulation of T cells (lymphocytes) that helps to stimulate the antibody production of B lymphocytes by antigens.
Hematocrit	The ratio of packed red blood cells to total blood volume in a centrifuged sample of blood, expressed as a percentage.
Heme	The iron-containing red pigment that, together with the protein globin, forms hemoglobin.
Hemoglobin	The pigment of red blood cells constituting about 33% of the cell volume that transports oxygen and carbon dioxide.
Hemopoiesis	The production of red blood cells.
Heparin	A mucopolysaccharide found in many tissues, but most abundantly in the lungs and liver, that is used medically as an anticoagulant.
Hepatic duct	A duct formed from the fusion of several bile ducts that drain bile from the liver. The hepatic duct merges with the cystic duct from the gallbladder to form the common bile duct.
Hepatic portal circulation	The return of venous blood from the digestive organs and spleen through a capillary network within the liver before draining into the heart.
Hepatitis	Inflammation of the liver.
Hepatopancreatic ampulla	A small, elevated area within the duodenum where the combined pancreatic and common bile duct empties; also called the ampulla of Vater.
Hering-Breuer reflex	A reflex in which distension of the lungs stimulates stretch receptors, which in turn act to inhibit further distension of the lungs.
Hermaphrodite	An organism having both testes and ovaries.
Heterochromatin	A condensed, inactive form of chromatin.
Hiatal hernia	A protrusion of an abdominal structure through the esophageal hiatus of the diaphragm into the thoracic cavity.
Hiatus	An opening or fissure; a foramen.
High-density lipoproteins (HDLs)	Combinations of lipids and proteins that migrate rapidly to the bottom of a test tube during centrifugation. HDLs are carrier proteins for lipids, such as cholesterol, that appear to offer some protection from atherosclerosis.
Hilum	A concave or depressed area where vessels or nerves enter or exit an organ; also called hilus.
Hinge joint	A type of synovial articulation characterized by a convex surface of one bone fitting into a concave surface of another such that movement is confined to one plane, as in the knee or interphalangeal joint.
Histamine	A compound secreted by tissue mast cells and other connective tissue cells that stimulates vasodilation and increases capillary permeability. It is responsible for many of the symptoms of inflammation and allergy.
Histology	Microscopic anatomy of the structure and function of tissues.
Homeostasis	The dynamic constancy of the internal environment, the maintenance of which is the principal function of physiological regulatory mechanisms. The concept of homeostasis provides a framework for understanding most physiological processes.
Homologous chromosomes	The matching pairs of chromosomes in a diploid cell.
Hormone	A chemical substance produced in an endocrine gland and secreted into the bloodstream to cause an effect in a specific target organ.
Humoral immunity	The form of acquired immunity in which antibody molecules are secreted in response to antigenic stimulation (as opposed to cell mediated immunity); also called antibody-mediated immunity.
Hyaline cartilage	A cartilage with a homogeneous matrix. It is the most common type, occurring at the articular ends of bones, in the trachea, and within the nose. Most of the bones in the body are formed from hyaline cartilage.
Hyaline membrane disease	A disease affecting premature infants who lack pulmonary surfactant, it is characterized by collapse of the alveoli (atelectasis) and pulmonary edema; also called respiratory distress syndrome.
Hydrocortisone	The principal corticosteroid hormone secreted by the adrenal cortex, with glucocorticoid action; also called cortisol.
Hydrophilic	Denoting a substance that readily absorbs water; literally, "water loving."
Hydrophobic	Denoting a substance that repels, and that is repelled by, water; "water fearing."
Hymen	A developmental remnant (vestige) of membranous tissue that partially covers the vaginal opening.
Hyperbaric oxygen	Oxygen gas present at greater than atmospheric pressure.
Hypercapnia	Excessive concentration of carbon dioxide in the blood.
Hyperextension	Extension beyond the normal anatomical position or 180∞.
Hyperglycemia	An abnormally increased concentration of glucose in the blood.
Hyperkalemia	An abnormally high concentration of potassium in the blood.
Hyperopia	A refractive disorder in which rays of light are brought to a focus behind the retina as a result of the eyeball being too short; also called farsightedness.
Hyperplasia	An increase in organ size due to an increase in cell numbers as a result of mitotic cell division (in contrast to hypertrophy).
Hyperpolarization	An increase in the negativity of the inside of a cell membrane with respect to the resting membrane potential.
Hypersensitivity	Another name for allergy; abnormal immune response that may be immediate (due to antibodies of the IgE class) or delayed (due to cell-mediated immunity).
Hypertension	Elevated or excessive blood pressure.
Hypertonic	Denoting a solution with a greater solute concentration and thus a greater osmotic pressure than plasma.
Hypertrophy	Growth of an organ due to an increase in the size of its cells (in contrast to hyperplasia).
Hyperventilation	A high rate and depth of breathing that results in a decrease in the blood carbon dioxide concentration to below normal.
Hypodermis	A layer of fat beneath the dermis of the skin.
Hyponychium	A thickened, supportive layer of stratum corneum at the distal end of a digit under the free edge of the nail.
Hypothalamic hormones	Hormones produced by the hypothalamus. These include antidiuretic hormone and oxytocin, which are secreted by the posterior pituitary, and both releasing and inhibiting hormones that regulate the secretions of the anterior pituitary.
Hypothalamo-hypophyseal portal system	A vascular system that transports releasing and inhibiting hormones from the hypothalamus to the anterior pituitary.
Hypothalamo-hypophyseal tract	The tract of nerve fibers (axons) that transports antidiuretic hormone and oxytocin from the hypothalamus to the posterior pituitary.
Hypothalamus	A portion of the forebrain within the diencephalon that lies below the thalamus, where it functions as an autonomic nerve centre and regulates the pituitary gland.
Hypovolemic shock	A rapid fall in blood pressure as a result of diminished blood volume.
Hypoxemia	A low oxygen concentration of the arterial blood.
Ileocecal valve	A modification of the mucosa at the junction of the small and large intestine that forms a one-way passage and prevents the backflow of food materials.
Ileum	The terminal portion of the small intestine between the jejunum and cecum.
Immediate hypersensitivity	Hypersensitivity (allergy) mediated by antibodies of the IgE class that results in the release of histamine and related compounds from tissue cells.
Immunization	The process of increasing one's resistance to pathogens. In active immunity a person is injected with antigens that stimulate the development of clones of specific B or T lymphocytes; in passive immunity a person is injected with antibodies produced by another organism.
Immunoassay	Any of a number of laboratory or clinical techniques that employ the specific binding between an antigen and its homologous antibody in order to identify and quantify a substance in a sample.
Immunoglobulins	Subclasses of the gamma globulin fraction of plasma proteins that have antibody functions, providing humoral immunity.
Immunosurveillance	The concept that the immune system recognizes and attacks malignant cells that produce antigens not recognized as "self." This function is believed to be cell mediated rather than humoral.
Implantation	The process by which a blastocyst attaches itself to and penetrates into the endometrium of the uterus.
Incus	The middle of three auditory ossicles within the middle-ear chamber; commonly called the anvil.
Inferior vena cava	A large systemic vein that collects blood from the body regions inferior to the level of the heart and returns it to the right atrium.
Infundibulum	The stalk that attaches the pituitary gland to the hypothalamus of the brain.
Ingestion	The process of taking food or liquid into the body by way of the oral cavity.
Inguinal	Pertaining to the groin region.
Inguinal canal	The circular passageway in the abdominal wall through which a testis descends into the scrotum.
Inhibin	A polypeptide hormone secreted by the testes that is believed to specifically exert negative feedback inhibition of FSH secretion from the anterior pituitary.
Inositol	A sugar like B-complex vitamin. Inositol triphosphate is believed to act as a second messenger in the action of some hormones.
Insertion	The more movable attachment of a muscle, usually more distal.
Inspiration	The act of breathing air into the alveoli of the lungs; also called inhalation.
Insula	A deep, paired cerebral lobe.
Insulin	A polypeptide hormone secreted by the beta cells of the pancreatic islets that promotes the anabolism of carbohydrates, fat, and protein. Insulin acts to promote the cellular uptake of blood glucose and, therefore, to lower the blood glucose concentration; insulin deficiency results in hyperglycemia and diabetes mellitus.
Integument	The skin; the largest organ of the body.
Intercalated disc	A thickened portion of the sarcolemma that extends across a cardiac muscle fibre, indicating the boundary between cells.
Intercellular substance	The matrix or material between cells that largely determines tissue types.
Interferons	A group of small proteins that inhibit the multiplication of viruses inside host cells and that also have antitumor properties.
Internal ear	The innermost portion or chamber of the ear, containing the cochlea and the vestibular organs.
Interneurons	Multipolar neurons interposed between sensory (afferent) and motor (efferent) neurons and confined entirely within the central nervous system; also called association neurons.
Interoceptors	Sensory receptors that respond to changes in the internal environment (as opposed to exteroceptors).
Interphase	The interval between successive cell divisions, during which time the chromosomes are in an extended state and are active in directing RNA synthesis.
Interstitial cells	Cells located in the interstitial tissue between adjacent convolutions of the seminiferous tubules of the testes; they secrete androgens (mainly testosterone); also called cells of Leydig.
Intervertebral disc	A pad of fibrocartilage located between the bodies of adjacent vertebrae.
Intestinal crypt	A simple tubular digestive gland opening onto the surface of the intestinal mucosa that secretes digestive enzymes; also called the crypt of Lieberkühn.
Intrafusal fibers	Modified muscle fibers that are encapsulated to form muscle spindle organs, which are muscle stretch receptors.
Intramembranous ossification	See membranous bone.
Intrapleural space	An actual or potential space between the visceral pleural membrane covering the lungs and the somatic pleural membrane lining the thoracic wall.
Intrinsic	Situated within or pertaining to internal origin.
Inulin	A polysaccharide of fructose, produced by certain plants, that is filtered by the human kidneys but neither reabsorbed nor secreted. The clearance rate of injected insulin is thus used to measure the glomerular filtration rate.
Inversion	A movement of the foot in which the sole is turned inward.
In vitro	Occurring outside the body, in a test tube or other artificial environment.
In vivo	Occurring within the body.
Ion	An atom or group of atoms that has either lost or gained electrons and thus has a net positive or a net negative charge.
Ionization	The dissociation of a solute to form ions.
Ipsilateral	On the same side (as opposed to contralateral).
IPSP	Inhibitory postsynaptic potential; hyperpolarization of the postsynaptic membrane in response to a particular neurotransmitter chemical, which makes it more difficult for the postsynaptic cell to attain a threshold level of depolarization required to produce action potentials. It is responsible for postsynaptic inhibition.
Iris	The pigmented portion of the vascular tunic of the eye that surrounds the pupil and regulates its diameter.
Ischemia	A rate of blood flow to an organ that is inadequate to supply sufficient oxygen and maintain aerobic respiration in that organ.
Islets of Langerhans	See pancreatic islets.
Isoenzymes	Enzymes, usually produced by different organs, that catalyze the same reaction but that differ from each other in amino acid composition.
Isometric contraction	Muscle contraction in which there is no appreciable shortening of the muscle.
Isotonic contraction	Muscle contraction in which the muscle shortens in length and maintains approximately the same amount of tension throughout the shortening process.
Isotonic solution	A solution having the same total solute concentration, osmolality, and osmotic pressure as the solution with which it is compared; a solution with the same solute concentration and osmotic pressure as plasma.
Isthmus	A narrow neck or portion of tissue connecting two structures.
Jaundice	A condition characterized by high blood bilirubin levels and staining of the tissues with bilirubin, which imparts a yellow colour to the skin and mucous membranes.
Jejunum	The middle portion of the small intestine, located between the duodenum and the ileum.
Joint capsule	The fibrous tissue that encloses the joint cavity of a synovial joint.
Keratin	An insoluble protein present in the epidermis and in epidermal derivatives, such as hair and nails.
Ketoacidosis	A type of metabolic acidosis resulting from the excessive production of ketone bodies, as in diabetes mellitus.
Ketogenesis	The production of ketone bodies.
Ketone bodies	The substances derived from fatty acids via acetyl coenzyme A in the liver; namely, acetone, acetoacetic acid, and b-hydroxybutyric acid. Ketone bodies are oxidized by skeletal muscles for energy.
Ketosis	An abnormal elevation in the blood concentration of ketone bodies that does not necessarily produce acidosis.
Kidney	One of a pair of organs of the urinary system that contains nephrons and that filters wastes from the blood in the formation of urine.
Kilocalorie	A unit of measurement equal to 1000 calories, which are units of heat (a kilocalorie is the amount of heat required to raise the temperature of 1 kilogram of water by 1 C∞). In nutrition, the kilocalorie is called a big calorie (Calorie).
Kinesiology	The study of body movement.
Klinefelter's syndrome	An abnormal condition of male sex characteristics due to the presence of an extra X chromosome (genotype XXY).
Knee	A region in the lower extremity between the thigh and the leg that contains a synovial hinge joint.
Krebs cycle	A cyclic metabolic pathway in the matrix of mitochondria by which the acetic acid part of acetyl CoA is oxidized and substrates provided for reactions that are coupled to the formation of ATP.
Kupffer cells	Phagocytic cells lining the sinusoids of the liver that are part of the body immunity system.
Labial frenulum	A longitudinal fold of mucous membrane that attaches the lips to the gum along the midline of both the upper and lower lip.
Labia majora	A portion of the external genitalia of a female consisting of two longitudinal folds of skin extending downward and backward from the mons pubis.
Labia minora	Two small folds of skin, devoid of hair and sweat glands, lying between the labia major of the external genitalia of a female.
Labyrinth	An intricate structure consisting of interconnecting passages (e.g., the bony and membranous labyrinths of the inner ear.
Lacrimal canaliculus	A drainage duct for tears, located at the medial corner of an eyelid. It conveys the tears medially into the nasolacrimal sac.
Lacrimal gland	A tear-secreting gland, located on the superior lateral portion of the eyeball underneath the upper eyelid.
Lactation	The production and secretion of milk by the mammary glands.
Lacteal	A small lymphatic duct associated with a villus of the small intestine.
Lactose	Milk sugar; a disaccharide of glucose and galactose.
Lactose intolerance	A disorder resulting in the inability to digest lactose because of an enzyme, lactase, deficiency. Symptoms include bloating, intestinal gas, nausea, diarrhea, and cramps.
Lacuna	A small, hollow chamber that houses an osteocyte in mature bone tissue or a chondrocyte in cartilage tissue.
Lambdoidal suture	The immovable joint in the skull between the parietal bones and the occipital bone.
Lamella	A concentric ring of matrix surrounding the central canal in an osteon of mature bone tissue.
Lamellated corpuscle	A sensory receptor for pressure, found in tendons, around joints, and in visceral organs; also called a pacinian corpuscle.
Lamina	A thin plate of bone that extends superiorly from the body of a vertebra to form either side of the arch of a vertebra.
Lanugo	Short, silky fetal hair, which may be present for a short time on a premature infant.
Large intestine	The last major portion of the GI tract, consisting of the cecum, colon, rectum, and anal canal.
Laryngopharynx	The inferior or lower portion of the pharynx in contact with the larynx. 686
Larynx	The structure located between the pharynx and trachea that houses the vocal cords; commonly called the voice box.
Lateral	Pertaining to the side; farther from the mid plane.
Lateral ventricle	A cavity within the cerebral hemisphere of the brain that is filled with cerebrospinal fluid.
L-dopa	Levodopa; a derivative of the amino acid tyrosine. It serves as the precursor for the neurotransmitter molecule dopamine and is given to patients with Parkinson's disease to stimulate dopamine production.
Leg	The portion of the lower extremity between the knee and ankle.
Lens	A transparent refractive organ of the eye positioned posterior to the pupil and iris.
Lesion	A wounded or damaged area.
Lesser omentum	A peritoneal fold of tissue extending from the lesser curvature of the stomach to the liver.
Leukocyte	A white blood cell; variant spelling, leucocyte.
Ligament	A tough cord or fibrous band of connective tissue that binds bone to bone to strengthen and provide flexibility to a joint. It also may support viscera.
Limbic system	A portion of the brain concerned with emotions and autonomic activity.
Linea alba	A vertical fibrous band extending down the anterior medial portion of the abdominal wall.
Lingual frenulum	A longitudinal fold of mucous membrane that attaches the tongue to the floor of the oral cavity.
Lipogenesis	The formation of fat or triglycerides.
Lipolysis	The hydrolysis of triglycerides into free fatty acids and glycerol.
Liver	A large visceral organ inferior to the diaphragm in the right hypochondriac region. The liver detoxifies the blood and modifies the blood plasma concentration of glucose, triglycerides, ketone bodies, and proteins.
Low-density lipoproteins	Plasma proteins that transport triglycerides and cholesterol. They are believed to contribute to arteriosclerosis.
Lower extremity	A lower appendage, including the hip, thigh, knee, leg, and foot.
Lumbar	Pertaining to the region of the loins.
Lumbar plexus	A network of nerves formed by the anterior branches of spinal nerves L1 through L4.
Lumen	The space within a tubular structure through which a substance passes.
Lung	One of the two major organs of respiration positioned within the thoracic cavity on either side of the mediastinum.
Lung surfactant	A mixture of lipoproteins (containing phospholipids) secreted by type II alveolar cells into the alveoli of the lungs. It lowers surface tension and prevents collapse of the lungs as occurs in hyaline membrane disease, in which surfactant is absent.
Lunula	The half-moon-shaped whitish area at the proximal portion of a nail.
Luteinizing hormone	(LH) A hormone secreted by the adenohypophysis (anterior lobe) of the pituitary gland that stimulates ovulation and the secretion of progesterone by the corpus luteum. It also influences mammary gland milk secretion in females and stimulates testosterone secretion by the testes in males.
Lymph (limf)	A clear, plasma like fluid that flows through lymphatic vessels.
Lymphatic system	The lymphatic vessels and lymph nodes.
Lymph node	A small, ovoid mass of reticular tissue located along the course of lymph vessels.
Lymphocyte	A type of white blood cell characterized by agranular cytoplasm. Lymphocytes usually constitute about 20% to 25% of the white blood cell count.
Lymphokines	A group of chemicals released from T cells that contribute to cell-mediated immunity.
Lysosomes	Organelles containing digestive enzymes and responsible for intracellular digestion.
Macromolecules	Large molecules; a term that usually refers to protein, RNA, and DNA.
Macrophage	A wandering phagocytic cell.
Macula lutea	A yellowish depression in the retina of the eye that contains the fovea centralis, the area of keenest vision.
Malignant	Threatening to life; virulent. Of a tumor, cancerous, tending to metastasize.
Malleus	The first of three auditory ossicles that attaches to the tympanum; commonly called the hammer.
Mammary gland	The gland of the female breast responsible for lactation and nourishment of the young.
Marrow	The soft connective tissue found within the inner cavity of certain bones that produces red blood cells.
Mast cell	A type of connective tissue cell that produces and secretes histamine and heparin and promotes local inflammation.
Mastication	The chewing of food.
Matrix	The intercellular substance of a tissue.
Maximal oxygen uptake	The maximum amount of oxygen that can be consumed by the body per unit time during heavy exercise.
Meatus	A passageway or opening into a structure.
Mechanoreceptor	A sensory receptor that responds to a mechanical stimulus.
Medial	Toward or closer to the mid plane of the body.
Mediastinum	The partition in the centre of the thorax between the two pleural cavities.
Medulla	The centre portion of an organ.
Medulla oblongata	A portion of the brain stem located between the spinal cord and the pons.
Medullary	The hollow core of the diaphysis of a long bone in which marrow is found.
Megakaryocyte	A bone marrow cell that gives rise to blood platelets.
Meiosis	A specialized type of cell division by which gametes or haploid sex cells are formed.
Meissner's corpuscle	See corpuscle of touch.
Melanin	A dark pigment found within the epidermis or epidermal derivatives of the skin.
Melanocyte	A specialized melanin-producing cell found in the deepest layer of the epidermis.
Melanoma	A dark, malignant tumor of the skin that frequently forms in moles.
Melatonin	A hormone secreted by the pineal gland that produces lightening of the skin in lower vertebrates and that may contribute to the regulation of gonadal function in mammals. Secretion follows a circadian rhythm and peaks at night.
Membrane potential	The potential difference or voltage that exists between the inner and outer sides of a cell membrane. It exists in all cells but is capable of being changed by excitable cells (neurons and muscle cells).
Membranous bone	Bone that forms from membranous connective tissue rather than from cartilage.
Membranous labyrinth	A system of communicating sacs and ducts within the bony labyrinth of the inner ear that includes the cochlea and vestibular apparatus. It is filled with endolymph and surrounded by perilymph and bone.
Menarche	The first menstrual discharge.
Ménière's disease	Deafness, tinnitus, and vertigo resulting from a disorder of the labyrinth.
Meninges	A group of three fibrous membranes covering the central nervous system, composed of the dura mater, arachnoid mater, and pia mater.
Menisci	Wedge-shaped fibrocartilages in certain synovial joints.
Menopause	The period marked by the cessation of menstrual periods in the human female.
Menstrual cycle	The rhythmic female reproductive cycle, characterized by changes in hormone levels and physical changes in the uterine lining.
Menstruation	The discharge of blood and tissue from the uterus at the end of the menstrual cycle.
Mesencephalic aqueduct	The channel that connects the third and fourth ventricles of the brain; also called the aqueduct of Sylvius.
Mesencephalon	The midbrain, which contains the corpora quadrigemina and the cerebral peduncles.
Mesenchyme	An embryonic connective tissue that can migrate, and from which all connective tissues arise.
Mesenteric patches	Clusters of lymph nodes on the walls of the small intestine; also called Peyer's patches.
Mesentery	A fold of peritoneal membrane that attaches an abdominal organ to the abdominal wall.
Mesoderm	The middle one of the three primary germ layers.
Mesothelium	A simple squamous epithelial tissue that lines body cavities and covers visceral organs; also called serosa.
Mesovarium	The peritoneal fold that attaches an ovary to the broad ligament of the uterus.
Messenger RNA	(mRNA) A type of RNA that contains a base sequence complementary to a part of the DNA that specifies the synthesis of a particular protein.
Metabolism	The sum total of the chemical changes that occur within a cell.
Metacarpus	The region of the hand between the wrist and the phalanges, including the five metacarpal bones that support the palm of the hand.
Metarteriole	A small blood vessel that emerges from an arteriole, passes through a capillary network, and empties into a venule.
Metastasis	The spread of a disease from one organ or body part to another.
Metatarsus	The region of the foot between the ankle and the phalanges that includes the five metatarsal bones.
Metencephalon	The most superior portion of the hindbrain that contains the cerebellum and the pons.
Micelles	Colloidal particles formed by the aggregation of many molecules.
Microglia	Small phagocytic cells found in the central nervous system.
Microvilli	Microscopic hairlike projections of cell membranes on certain epithelial cells.
Micturition	The process of voiding urine; also called urination.
Midbrain	The portion of the brain between the pons and the forebrain.
Middle ear	The middle of the three portions of the ear that contains the three auditory ossicles.
Midsagittal plane	A plane that divides the body into equal right and left halves; also called the median plane or mid plane.
Mineralocorticoids	Steroid hormones of the adrenal cortex (corticosteroids) that regulate electrolyte balance.
Mitochondria	Cytoplasmic organelles that serve as sites for the production of most of the cellular energy; the so-called powerhouses of the cell.
Mitosis	The process of cell division that results in two identical daughter cells, containing the same number of chromosomes.
Mitral valve	The left atrioventricular heart valve; also called the bicuspid valve.
Mixed nerve	A nerve that contains both motor and sensory nerve fibers.
Molal	Pertaining to the number of moles of solute per kilogram of solvent.
Molar	Pertaining to the number of moles of solute per litre of solution.
Mole	The number of grams of a chemical that is equal to its formula weight (atomic weight for an element or molecular weight for a compound).
Monoclonal antibodies	Identical antibodies derived from a clone of genetically identical plasma cells.
Monocyte	A phagocytic type of white blood cell, normally constituting about 3% to 8% of the white blood cell count.
Monomer	A single molecular unit of a longer, more complex molecule. Monomers are joined together to form dimers, trimers, and polymers; the hydrolysis of polymers eventually yields separate monomers.
Monosaccharide	The monomer of the more complex carbohydrates, examples of which include glucose, fructose, and galactose; also called a simple sugar.
Mons pubis	A fatty tissue pad covering the symphysis pubis and covered by pubic hair in the female.
Morula	An early stage of embryonic development characterized by a solid ball of cells.
Motile	Capable of self-propelled movement.
Motor area	A region of the cerebral cortex from which motor impulses to muscles or glands originate.
Motor nerve	A nerve composed of motor nerve fibers.
Motor neuron	A nerve cell that conducts action potentials away from the central nervous system and innervates effector organs (muscle and glands). It forms the anterior roots of the spinal nerves; also called an efferent neuron.
Motor unit	A single motor neuron and the muscle fibers it innervates.
Mucosa	A mucous membrane that lines cavities and tracts opening to the exterior.
Mucous cell	See goblet cell.
Mucous membrane	A thin sheet consisting of layers of visceral organs that include the lining epithelium, submucosal connective tissue, and (in some cases) a thin layer of smooth muscle (the muscularis mucosa).
Multipolar neuron	A nerve cell with many processes originating from the cell body.
Muscle	A major type of tissue adapted to contract. The three kinds of muscle are cardiac, smooth, and skeletal.
Muscle spindles	Sensory organs within skeletal muscles composed of intrafusal fibers. They are sensitive to muscle stretch and provide a length detector within muscles.
Muscularis	A muscular layer or tunic of an organ, composed of smooth muscle tissue.
Myelencephalon	The posterior portion of the hindbrain that contains the medulla oblongata.
Myelin	A lipoprotein material that forms a sheath-like covering around nerve fibers.
Myelin sheath	A sheath surrounding axons formed by successive wrappings of a neuroglial cell membrane. Myelin sheaths are formed by neurolemmocytes in the peripheral nervous system and by oligodendrocytes within the central nervous system.
Myenteric plexus	A network of sympathetic and parasympathetic nerve fibers located in the muscularis tunic of the small intestine; also called the plexus of Auerbach.
Myocardial infarction	An area of necrotic tissue in the myocardium that is filled in by scar (connective) tissue.
Myocardium	The cardiac muscle layer of the heart.
Myofibril	A bundle of contractile fibers within muscle cells.
Myogenic	Originating within muscle cells; used to describe self-excitation by cardiac and smooth muscle cells.
Myoglobin	A molecule composed of globin protein and heme pigment. It is related to hemoglobin but contains only one subunit (instead of the four in hemoglobin) and is found in skeletal and cardiac muscle cells where it serves to store oxygen.
Myogram	A recording of electrical activity within a muscle.
Myology	The science or study of muscle structure and function.
Myometrium	The layer or tunic of smooth muscle within the uterine wall.
Myoneural junction	The site of contact between an axon of a motor neuron and a muscle fibre.
Myopia	A visual defect in which objects may be seen distinctly only when very close to the eyes; also called nearsightedness.
Myosin	A thick myofilament protein that together with actin causes muscle contraction.
Myxedema	A type of edema associated with hypothyroidism. It is characterized by the accumulation of mucoproteins in tissue fluid.
NAD	Nicotinamide adenine dinucleotide; a coenzyme derived from niacin that helps to transport electrons from the Krebs cycle to the electron-transport chain within mitochondria.
Nail	A hardened, keratinized plate that develops from the epidermis and forms a protective covering on the surface of the distal phalanges of fingers and toes.
Naloxone	A drug that antagonizes the effects of morphine and endorphins.
Nasal cavity	A mucosa-lined space above the oral cavity, divided by a nasal septum. It is the first chamber of the respiratory system.
Nasal concha	A scroll-like bone extending medially from the lateral wall of the nasal cavity; also called a turbinate bone.
Nasal septum	A bony and cartilaginous partition that separates the nasal cavity into two portions.
Nasopharynx	The first or uppermost chamber of the pharynx, positioned posterior to the nasal cavity and extending down to the soft palate.
Natriuretic	An agent that promotes the excretion of sodium in the urine. Atrial natriuretic hormone has this effect.
Neck	1.Any constricted portion, such as the neck of an organ. 2.The cervical region of the body between the head and thorax.
Necrosis	Cellular death or tissue death due to disease or trauma.
Negative feedback	A mechanism in the body for maintaining a state of internal constancy, or homeostasis; effectors are activated by changes in the internal environment, and the actions of the effectors serve to counteract these changes and maintain a state of balance.
Neonatal	The stage of life from birth to the end of 4 weeks.
Neoplasm	A new, abnormal growth of tissue, as in a tumor.
Nephron	The functional unit of the kidney, consisting of a glomerulus, convoluted tubules, and a nephron loop.
Nerve	A bundle of nerve fibers outside the central nervous system.
Neurilemma	A thin, membranous covering surrounding the myelin sheath of a nerve fibre.
Neurofibril node	A gap in the myelin sheath of a nerve fibre; also called a node of Ranvier.
Neuroglia	Specialised supportive cells of the central nervous system.
Neurohypophysis	The posterior lobe of the pituitary gland derived from the brain. Its major secretions include antidiuretic hormone (ADH), also called vasopressin, and oxytocin, produced in the hypothalamus.
Neuro lemmocyte	A specialized neuroglia cell that surrounds an axon fibre of a peripheral nerve and forms the neurilemmal sheath; also called a Schwann cell.
Neuron	The structural and functional unit of the nervous system, composed of a cell body, dendrites, and an axon; also called a nerve cell.
Neurotransmitter	A chemical contained in synaptic vesicles in nerve endings that is released into the synaptic cleft, where it stimulates the production of either excitatory or inhibitory postsynaptic potentials.
Neutrons	Electrically neutral particles that exist together with positively charged protons in the nucleus of atoms.
Neutrophil	A type of phagocytic white blood cell, normally constituting about 60% to 70% of the white blood cell count.
Nexus	A bond between members of a group; the type of bonds present in single-unit smooth muscles.
Nidation	Implantation of the blastocyst into the endometrium of the uterus.
Nipple	A dark pigmented, rounded projection at the tip of the breast.
Norepinephrine	A catecholamine released as a neurotransmitter from postganglionic sympathetic nerve endings and as a hormone (together with epinephrine) from the adrenal medulla.
Notochord	A flexible rod of tissue that extends the length of the back of an embryo.
Nucleolus	A dark-staining area within a cell nucleus; the site where ribosomal RNA is produced.
Nucleoplasm	The protoplasmic contents of the nucleus of a cell.
Nucleotide	The subunit of DNA and RNA macromolecules. Each nucleotide is composed of a nitrogenous base (adenine, guanine, cytosine, and thymine or uracil); a sugar (deoxyribose or ribose); and a phosphate group.
Nucleus	A spheroid body within a cell that contains the genetic factors of the cell.
Nucleus pulposus	The soft, pulpy core of an intervertebral disc; a remnant of the notochord.
Nystagmus	Involuntary oscillary movements of the eye.
Obese	Excessively fat.
Olfactory	Pertaining to the sense of smell.
Olfactory bulb	An aggregation of sensory neurons of an olfactory nerve, lying inferior to the frontal lobe of the cerebrum on either lateral side of the crista galli of the ethmoid bone.
Olfactory tract	The olfactory sensory tract of axons that conveys impulses from the olfactory bulb to the olfactory portion of the cerebral cortex.
Oligodendrocyte	A type of neuroglial cell concerned with the formation of the myelin of nerve fibers within the central nervous system.
Oncology	The study of tumors.
Oncotic pressure	The colloid osmotic pressure of solutions produced by proteins. In plasma, it serves to counterbalance the outward filtration of fluid from capillaries due to hydrostatic pressure.
Oocyte	A developing egg cell.
Oogenesis	The process of female gamete formation.
Opsonization	The process by which antibodies enhance the ability of phagocytic cells to attack bacteria.
Optic	Pertaining to the eye.
Optic chiasma	An X-shaped structure on the inferior aspect of the brain, anterior to the pituitary gland, where there is a partial crossing over of fibers in the optic nerves; also called the optic chiasm.
Optic disc	A small region of the retina where the fibers of the ganglion neurons exit from the eyeball to form the optic nerve; also called the blind spot.
Optic tract	A bundle of sensory axons located between the optic chiasma and the thalamus that functions to convey visual impulses from the photoreceptors within the eye.
Oral	Pertaining to the mouth.
Ora serrata	The jagged peripheral margin of the retina.
Organ	A structure consisting of two or more tissues that performs a specific function.
Organelle	A minute living structure of a cell with a specific function.
Organism	An individual living creature.
Orifice	An opening into a body cavity or tube.
Origin	The place of muscle attachment-usually the more stationary point or the proximal bone; opposite the insertion.
Oropharynx	The second portion of the pharynx, located posterior to the oral cavity and extending from the soft palate to the hyoid bone.
Osmolality	A measure of the total concentration of a solution; the number of moles of solute per kilogram of solvent.
Osmoreceptors	Sensory neurons that respond to changes in the osmotic pressure of the surrounding fluid.
Osmosis	The passage of solvent (water) from a more dilute to a more concentrated solution through a membrane that is more permeable to water than to the solute.
Osmotic pressure	A measure of the tendency of a solution to gain water by osmosis when separated by a membrane from pure water. Directly related to the osmolality of the solution, it is the pressure required to just prevent osmosis.
Osseous tissue	Bone tissue.
Ossicle	One of the three bones of the middle ear; also called the auditory ossicle.
Ossification	The process of bone tissue formation.
Osteoblast	A bone-forming cell.
Osteoclast	A cell that causes erosion and resorption of bone tissue.
Osteocyte	A mature bone cell.
Osteology	The study of the structure and function of bone and the entire skeleton.
Osteomalacia	Softening of bones due to a deficiency of vitamin D and calcium.
Osteon	A group of osteocytes and concentric lamellae surrounding a central canal, constituting the basic unit of structure in osseous tissue; also called a haversian system.
Osteoporosis	Demineralization of bone, seen most commonly in postmenopausal women and patients who are inactive or paralyzed. It may be accompanied by pain, loss of stature, and other deformities and fractures.
Otoliths	Small, hardened particles of calcium carbonate in the saccule and utricle of the inner ear, associated with the receptors of equilibrium; also called statoconia.
Outer ear	The outer portion of the ear, consisting of the auricle and the external auditory canal.
Oval window	An oval opening in the bony wall between the middle and inner ear, into which the footplate of the stapes fits; also called the vestibular window.
Ovarian follicle	A developing ovum and its surrounding epithelial cells.
Ovarian ligament	A cordlike connective tissue that attaches the ovary to the uterus.
Ovary	The female gonad in which ova and certain sexual hormones are produced.
Oviduct	The tube that transports ova from the ovary to the uterus; also called the uterine tube or fallopian tube.
Ovulation	The rupture of an ovarian (graafian) follicle with the release of an ovum.
Ovum	A secondary oocyte capable of developing into a new individual when fertilized by a spermatozoon.
Oxidative phosphorylation	The formation of ATP using energy derived from electron transport to oxygen. It occurs in the mitochondria.
Oxidizing agent	An atom that accepts electrons in an oxidation-reduction reaction.
Oxyhemoglobin	A compound formed by the bonding of molecular oxygen to hemoglobin.
Oxyhemoglobin saturation	The ratio, expressed as a percentage, of the amount of oxyhemoglobin relative to the total amount of hemoglobin in blood.
Oxytocin	One of the two hormones produced in the hypothalamus and secreted by the posterior pituitary (the other hormone is vasopressin). Oxytocin stimulates the contraction of uterine smooth muscles and promotes milk ejection in females.
Pacemaker	A group of cells that has the fastest spontaneous rate of depolarization and contraction in a mass of electrically coupled cells; in the heart, this is the sinoatrial, or SA, node.
PAH	Para-aminohippuric acid; a substance used to measure total renal plasma flow because its clearance rate is equal to the total rate of plasma flow to the kidneys. PAH is filtered and secreted but not reabsorbed by the renal nephrons.
Palate	The roof of the oral cavity.
Palatine	Pertaining to the palate.
Palmar	Pertaining to the palm of the hand.
Palpebra	An eyelid.
Pancreas	A mixed organ in the abdominal cavity that secretes pancreatic juices into the GI tract and insulin and glucagon into the blood.
Pancreatic duct	A drainage tube that carries pancreatic juice from the pancreas into the duodenum of the hepatopancreatic ampulla.
Pancreatic islets	A cluster of cells within the pancreas that forms the endocrine portion and secretes insulin and glucagon; also called islets of Langerhans.
Papillary muscle	Muscular projections from the ventricular walls of the heart to which the chordea tendineae are attached.
Paranasal sinus	An air chamber lined with a mucous membrane that communicates with the nasal cavity.
Parasympathetic	Pertaining to the division of the autonomic nervous system concerned with activities that, in general, inhibit or oppose the physiological effects of the sympathetic nervous system.
Parathyroid hormone	(PTH) A polypeptide hormone secreted by the parathyroid glands. PTH acts to raise the blood Ca++ levels primarily by stimulating reabsorption of bone.
Parathyroids	Small endocrine glands embedded on the posterior surface of the thyroid glands that are concerned with calcium metabolism.
Parietal	Pertaining to a wall of an organ or cavity.
Parietal pleura	The thin serous membrane attached to the thoracic walls of the pleural cavity.
Parkinson's disease	A tremor of the resting muscles and other symptoms caused by inadequate dopamine-producing neurons in the basal nuclei of the cerebrum; also called paralysis agitans.
Parotid gland	One of the paired salivary glands located on the side of the face over the masseter muscle just anterior to the ear and connected to the oral cavity through a salivary duct.
Parturition	The process of giving birth; childbirth.
Passive immunity	Specific immunity granted by the administration of antibodies made by another organism.
Pathogen	Any disease-producing microorganism or substance.
Pectoral	Pertaining to the chest region.
Pectoral girdle	The portion of the skeleton that supports the upper extremities.
Pedicle	The portion of a vertebra that connects and attaches the lamina to the body.
Pelvic	Pertaining to the pelvis.
Pelvic girdle	The portion of the skeleton to which the lower extremities are attached.
Pelvis	A basin like bony structure formed by the sacrum and ossa coxae.
Penis	The male organ of copulation, used to introduce sperm into the female vagina and through which urine passes during urination.
Pennate	Pertaining to a skeletal muscle fiber arrangement in which the fibers are attached to tendinous slips in a featherlike pattern.
Pepsin	The protein-digesting enzyme secreted in gastric juice.
Peptic ulcer	An injury to the mucosa of the esophagus, stomach, or small intestine due to the action of acidic gastric juice.
Perforating canal	A minute duct through compact bone by which blood vessels and nerves penetrate to the central canal of an osteon; also called Volkmann's canal.
Pericardium	A protective serous membrane that surrounds the heart.
Perichondrium	A toughened connective sheet that covers some kinds of cartilage.
Perikaryon	The cell body of a neuron.
Perilymph	A fluid of the inner ear that provides a liquid-conducting medium for the vibrations involved in hearing and the maintenance of equilibrium.
Perimysium	Fascia (connective tissue) surrounding a bundle of muscle fibers.
Perineum	The floor of the pelvis, which is the region between the anus and the symphysis pubis. It is the region that contains the external genitalia.
Perineurium	Connective tissue surrounding a bundle of nerve fibers.
Periodontal membrane	A fibrous connective tissue lining the dental alveoli.
Periosteum	A fibrous connective tissue covering the outer surface of bone.
Peripheral nervous system	The nerves and ganglia of the nervous system that lie outside of the brain and spinal cord; PNS.
Peristalsis	Rhythmic contractions of smooth muscle in the walls of various tubular organs by which the contents are forced onward.
Peritoneum	The serous membrane that lines the abdominal cavity and covers the abdominal visceral organs. .
pH	A measure of the relative acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions. The pH scale in common use ranges from 0 to 14. Solutions with a pH lower than 7 are acidic and those with a higher pH are basic.
Phagocytosis	Cellular eating; the ability of some cells (such as white blood cells) to engulf large particles (such as bacteria) and digest these particles by merging the food vacuole in which they are contained with a lysosome containing digestive enzymes.
Phalanx	A bone of a finger or toe.
Pharynx	The organ of the digestive system and respiratory system located at the back of the oral and nasal cavities that extends to the larynx anteriorly and to the esophagus posteriorly; also called the throat.
Photoreceptor	A sensory nerve ending that responds to the stimulation of light.
Physiology	The science that deals with the study of body functions.
Pia mater	The innermost meninx that is in direct contact with the brain and spinal cord.
Pineal gland	A small cone-shaped gland located in the roof of the third ventricle.
Pinna	The outer, fleshy portion of the external ear; also called the auricle.
Pinocytosis	Cell drinking; invagination of the cell membrane forming narrow channels that pinch off into vacuoles. This allows for cellular intake of extracellular fluid and dissolved molecules.
Pituitary gland	A small, pea-shaped endocrine gland situated on the interior surface of the diencephalonic region of the brain, consisting of anterior and posterior lobes; also called the hypophysis.
Pivot joint	A synovial joint in which the rounded head of one bone articulates with the depressed cup of another to permit a rotational type of movement.
Placenta	The organ of metabolic exchange between the mother and the fetus.
Plantar	Pertaining to the sole of the foot.
Plasma	The fluid, extracellular portion of circulating blood.
Plasma cells	Cells derived from B lymphocytes that produce and secrete large amounts of antibodies. They are responsible for humoral immunity.
Platelets	Small fragments of specific bone marrow cells that function in blood coagulation; also called thrombocytes.
Pleural	Pertaining to the serous membranes associated with the lungs.
Pleural cavity	The potential space between the visceral pleura and parietal pleura.
Pleural membranes	Serous membranes that surround the lungs and provide protection and compartmentalization.
Plexus	A network of interlaced nerves or vessels.
Plicae circulares	Deep folds within the wall of the small intestine that increase the absorptive surface area.
Pneumotaxic area	The region of the respiratory control centre located in the pons of the brain.
Polar body	A small daughter cell formed by meiosis that degenerates in the process of oocyte production.
Polar molecule	A molecule in which the shared electrons are not evenly distributed, so that one side of the molecule is negatively (or positively) charged in comparison with the other side. Polar molecules are soluble in polar solvents, such as water.
Polydipsia	Excessive thirst.
Polymer	A large molecule formed by the combination of smaller subunits, or monomers.
Polymorphonuclear leukocyte	A granular leukocyte containing a nucleus with a number of lobes connected by thin, cytoplasmic strands. This type includes neutrophils, eosinophils, and basophils.
Polypeptide	A chain of amino acids connected by covalent bonds called peptide bonds. A very large polypeptide is called a protein.
Polysaccharide	A carbohydrate formed by covalent bonding of numerous monosaccharides. Examples include glycogen and starch.
Polyuria	Excretion of an excessively large volume of urine in a given period.
Pons	The portion of the brain stem just above the medulla oblongata and anterior to the cerebellum.
Popliteal	Pertaining to the concave region on the posterior aspect of the knee.
Posterior	Toward the back; also called dorsal.
Posterior pituitary	See neurohypophysis.
Posterior root	An aggregation of sensory neuron fibers lying between a spinal nerve and the posterolateral aspect of the spinal cord; also called the dorsal root or sensory root.
Posterior root ganglion	A cluster of cell bodies of sensory neurons located along the posterior root of a spinal nerve.
Postganglionic neuron	The second neuron in an autonomic motor pathway. Its cell body is outside the central nervous system and it terminates at an effector organ.
Postnatal	After birth.
Postsynaptic inhibition	The inhibition of a postsynaptic neuron by axon endings that release a neurotransmitter that induces hyperpolarization (inhibitory postsynaptic potentials).
Preganglionic neuron	The first neuron in an autonomic motor pathway. Its cell body is inside the central nervous system and it terminates on a postganglionic neuron.
Pregnancy	A condition in which a female is carrying a developing offspring within the body.
Prenatal	Pertaining to the period of offspring development during pregnancy; before birth.
Prepuce	A fold of loose, retractable skin covering the glans of the penis or clitoris; also called the foreskin.
Presynaptic inhibition	Neural inhibition in which axoaxonic synapses inhibit the release of neurotransmitter chemicals from the presynaptic axon terminal.
Prolactin	A hormone secreted by the anterior pituitary that, in conjunction with other hormones, stimulates lactation in the postpartum female. It may also participate (along with the gonadotropins) in regulating gonadal function in some mammals.
Pronation	A rotational movement of the forearm in which the palm of the hand is turned posteriorly.
Proprioceptor	A sensory nerve ending that responds to changes in tension in a muscle or tendon.
Prostaglandin	Any of a family of fatty acids that have numerous autocrine regulatory functions, including the stimulation of uterine contractions and of gastric acid secretion and the promotion of inflammation.
Prostate	A walnut-shaped gland surrounding the male urethra just below the urinary bladder that secretes an additive to seminal fluid during ejaculation.
Prosthesis	An artificial device to replace a diseased or worn body part.
Proton	A unit of positive charge in the nucleus of atoms.
Protoplasm	A general term for the colloidal complex of protein that constitutes the living material of a cell. It includes cytoplasm and nucleoplasm.
Protraction	The movement of a body part, such as the mandible, forward on a plane parallel with the ground; the opposite of retraction.
Proximal	Closer to the mid-plane of the body or to the origin of an appendage; the opposite of distal.
Pseudohermaphrodite	An individual with some of the physical characteristics of both sexes, but who lacks functioning gonads of both sexes; a true hermaphrodite has both testes and ovaries.
Pseudopods	Foot-like extensions of the cytoplasm that enable some cells (with amoeboid motion) to move across a substrate. Pseudopods are also used to surround food particles in the process of phagocytosis.
Ptyalin	An enzyme in saliva that catalyses the hydrolysis of starch into smaller molecules; also called salivary amylase.
Puberty	The period of development in which the reproductive organs become functional.
Pulmonary	Pertaining to the lungs.
Pulmonary circulation	The system of blood vessels from the right ventricle of the heart to the lungs that transports deoxygenated blood and returns oxygenated blood from the lungs to the left atrium of the heart.
Pulp cavity	A cavity within the centre of a tooth that contains blood vessels, nerves, and lymphatics.
Pupil	The opening through the iris that permits light to enter the posterior cavity of the eyeball and be refracted by the lens through the vitreous chamber.
Purkinje fibers	See conduction myofibers.
Pyloric sphincter	A modification of the muscularis tunic between the stomach and the duodenum that functions to regulate the food material leaving the stomach.
Pyramid	Any of several structures that have a pyramidal shape (e.g., the renal pyramids in the kidney and the medullary pyramids on the anterior surface of the brain).
Pyrogen	A fever-producing substance.
QRS complex	The principal deflection of an electrocardiogram that is produced by depolarization of the ventricles.
Ramus	A branch of a bone, artery, or nerve.
Raphe	A ridge or a seam-like structure between two similar parts of a body organ, as in the scrotum.
Receptor	A sense organ or a specialized distal end of a sensory neuron that receives stimuli from the environment.
Rectum	The terminal portion of the GI tract, between the sigmoid colon and the anal canal.
Red marrow	A tissue that forms blood cells, located in the medullary cavity of certain bones.
Red nucleus	An aggregation of grey matter of a reddish colour located in the upper portion of the midbrain. It sends fibers to certain brain tracts.
Reduced hemoglobin	Hemoglobin with iron in the reduced ferrous state. It is able to bond with oxygen but is not combined with oxygen. Also called deoxyhemoglobin.
Reducing agent	An electron donor in a coupled oxidation-reduction reaction.
Reflex	A rapid involuntary response to a stimulus.
Reflex arc	The basic conduction pathway through the nervous system, consisting of a sensory neuron, an association neuron, and a motor neuron.
Regional anatomy	The division of anatomy concerned with structural arrangement in specific areas of the body, such as the head, neck, thorax, or abdomen.
Renal	Pertaining to the kidney.
Renal corpuscle	The portion of the nephron consisting of the glomerulus and a glomerular capsule; also called the malpighian corpuscle.
Renal cortex	The outer portion of the kidney, primarily vascular.
Renal medulla	The inner portion of the kidney, including the renal pyramids and renal columns.
Renal pelvis	The inner cavity of the kidney formed by the expanded ureter and into which the calyces open.
Renal plasma clearance rate	The millilitres of plasma cleared of a particular solute per minute by the excretion of that solute in the urine. If there is no reabsorption or secretion of that solute by the nephron tubules, the plasma clearance rate is equal to the glomerular filtration rate.
Renal pyramid	A triangular structure within the renal medulla composed of nephron loops and the collecting ducts.
Repolarization	The reestablishment of the resting membrane potential after depolarization has occurred.
Respiration	The exchange of gases between the external environment and the cells of an organism.
Respiratory acidosis	A lowering of the blood pH to below 7.35 due to accumulation of CO2 as a result of hypoventilation.
Respiratory alkalosis	A rise in blood pH to above 7.45 due to excessive elimination of blood CO2 as a result of hyperventilation.
Respiratory centre	The structure or portion of the brain stem that regulates the depth and rate of breathing.
Respiratory distress syndrome	A lung disease of the newborn, most frequently occurring in premature infants, that is caused by abnormally high alveolar surface tension as a result of a deficiency in lung surfactant; also called hyaline membrane disease.
Respiratory membrane	A thin, moistened membrane within the lungs, composed of an alveolar portion and a capillary portion, through which gaseous exchange occurs.
Rete testis	A network of ducts in the centre of the testis associated with the production of spermatozoa.
Reticular formation	A network of nervous tissue fibers in the brain stem that arouses the higher brain centres.
Retina	The principal portion of the internal tunic of the eyeball that contains the photoreceptors.
Retraction	The movement of a body part, such as the mandible, backward on a plane parallel with the ground; the opposite of protraction.
Retroperitoneal	Positioned behind the parietal peritoneum.
Rhodopsin	A pigment in rod cells that undergoes a photochemical dissociation in response to light and in so doing stimulates electrical activity in the photoreceptors.
Rhythmicity area	A portion of the respiratory control centre located in the medulla oblongata that controls inspiratory and expiratory phases.
Ribosome	A cytoplasmic organelle composed of protein and RNA in which protein synthesis occurs.
Rickets	A condition caused by a deficiency of vitamin D and associated with an interference of the normal ossification of bone.
Right lymphatic duct	A major vessel of the lymphatic system that drains lymph from the upper right portion of the body into the right subclavian vein.
Rigor mortis	The stiffening of a dead body due to the depletion of ATP and the production of rigor complexes between actin and myosin in muscles.
RNA	Ribonucleic acid; a nucleic acid consisting of the nitrogenous bases adenine, guanine, cytosine, and uracil; the sugar ribose; and phosphate groups. There are three types of RNA found in cytoplasm: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
Rod	A photoreceptor in the retina of the eye that is specialized for colourless, dim-light vision.
Root canal	The hollow, tubular extension of the pulp cavity into the root of the tooth that contains vessels and nerves.
Rotation	The movement of a bone around its own longitudinal axis.
Round window	A round, membrane-covered opening between the middle and inner ear, directly below the oval window; also called the cochlear window.
Rugae	The folds or ridges of the mucosa of an organ.
Saccadic eye movements	Very rapid eye movements that occur constantly and that change the focus on the retina from one point to another.
Saccule	A saclike cavity in the membranous labyrinth inside the vestibule of the inner ear that contains a vestibular organ for equilibrium.
Sacral	Pertaining to the sacrum.
Sacral plexus	A network of nerve fibers that arises from spinal nerves L4 through S3. Nerves arising from the sacral plexus merge with those from the lumbar plexus to form the lumbosacral plexus and supply the lower extremity.
Saddle joint	A synovial joint in which the articular surfaces of both bones are concave in one plane and convex or saddle shaped, in the other plane, such as in the distal carpometacarpal joint of the thumb.
Sagittal plane	A vertical plane, running parallel to the midsagittal plane, that divides the body into unequal right and left portions.
Salivary gland	An accessory digestive gland that secretes saliva into the oral cavity.
Saltatory conduction	The rapid passage of action potentials from one node of Ranvier (neurofibril node) to another in myelinated axons.
Sarcolemma	The cell membrane of a muscle fiber.
Sarcomere	The portion of a striated muscle fiber between the two adjacent Z lines that is considered the functional unit of a myofibril.
Sacroplasm	The cytoplasm within a muscle fiber.
Sarcoplasmic reticulum	The smooth or agranular endoplasmic reticulum of skeletal muscle cells. It surrounds each myofibril and stores Ca++ when the muscle is at rest.
Scala tympani	The lower channel of the cochlea that is filled with perilymph.
Scala vestibuli	The upper channel of the cochlea that is filled with perilymph.
Sclera	The outer white layer of fibrous connective tissue that forms the protective covering of the eyeball.
Scleral venous sinus	A circular venous drainage for the aqueous humor from the anterior chamber; located at the junction of the sclera and the cornea; also called the canal of Schlemm.
Scrotum	A pouch of skin that contains the testes and their accessory organs.
Sebaceous gland	An exocrine gland of the skin that secretes sebum.
Sebum	An oily, waterproofing secretion of the sebaceous glands.
Second messenger	A molecule or ion whose concentration within a target cell is increased by the action of a regulatory compound (e.g., a hormone or neurotransmitter) and which stimulates the metabolism of that target cell in a way that mediates the intracellular effects of that regulatory compound.
Secretin	A polypeptide hormone secreted by the small intestine in response to acidity of the intestinal lumen. Along with cholecystokinin, secretin stimulates the secretion of pancreatic juice into the small intestine.
Semen	The thick, whitish secretion of the reproductive organs of the male, consisting of spermatozoa and additives from the prostate and seminal vesicles.
Semicircular canals	Tubule channels within the inner ear that contain receptors for equilibrium.
Semilunar valve	Crescent- or half-moon-shaped heart valves positioned at the entrances to the aorta and the pulmonary trunk.
Seminal vesicles	A pair of accessory male reproductive organs lying posterior and inferior to the urinary bladder that secrete additives to spermatozoa into the ejaculatory ducts.
Seminiferous tubules	Numerous small ducts in the testes, where spermatozoa are produced.
Semipermeable membrane	A membrane with pores of a size that permits the passage of solvent and some solute molecules while restricting the passage of other solute molecules.
Senescence	The process of aging.
Sensory area	A region of the cerebral cortex that receives and interprets sensory nerve impulses.
Sensory neuron	A nerve cell that conducts an impulse from a receptor organ to the central nervous system; also called an afferent neuron.
Septum	A membranous or fleshy wall dividing two cavities.
Serous membrane	An epithelial and connective tissue membrane that lines body cavities and covers visceral organs within these cavities; also called serosa.
Sertoli cells	See sustentacular cells.
Serum	Blood plasma with the clotting elements removed.
Sesamoid bone	A membranous bone formed in a tendon in response to joint stress (e.g., the patella).
Sex chromosomes	The X and Y chromosomes; the unequal pairs of chromosomes involved in sex determination (which is based on the presence or absence of a Y chromosome). Females lack a Y chromosome and normally have the genotype XX; males have a Y chromosome and normally have the genotype XY.
Shock	As it relates to the cardiovascular system, this term refers to a rapid, uncontrolled fall in blood pressure, which in some cases becomes irreversible and leads to death.
Shoulder	The region of the body where the humerus articulates with the scapula.
Sickle-cell anemia	A hereditary, autosomal recessive trait that occurs primarily in people of African ancestry, in which it evolved apparently as a protection (in the carrier state) against malaria. In the homozygous state, hemoglobin S is made instead of hemoglobin A; this leads to the characteristic sickling of red blood cells, hemolytic anemia, and organ damage.
Sigmoid colon	The S-shaped portion of the large intestine between the descending colon and the rectum.
Sinoatrial node	A mass of specialized cardiac tissue in the wall of the right atrium that initiates the cardiac cycle; the SA node; also called the pacemaker.
Sinus	A cavity or hollow space within a body organ, such as a bone.
Sinusoid	A small, blood-filled space in certain organs, such as the spleen or liver.
Skeletal muscle	A specialized type of multinucleated muscle tissue that occurs in bundles, has cross bands of proteins, and contracts in either a voluntary or involuntary fashion.
Sleep apnea	A temporary cessation of breathing during sleep, usually lasting for several seconds.
Sliding filament theory	The theory that the thick and thin filaments of a myofibril slide past each other during muscle contraction, while maintaining their initial length.
Small intestine	The portion of the GI tract between the stomach and the cecum whose function is the absorption of food nutrients.
Smooth muscle	A specialized type of nonstriated muscle tissue composed of fusiform, single-nucleated fibers. It contracts in an involuntary, rhythmic fashion within the walls of visceral organs.
Sodium/potassium pump	An active transport carrier with ATPase enzymatic activity that acts to accumulate K+ within cells and extrude Na+ from cells, thus maintaining gradients for these ions across the cell membrane.
Soft palate	The fleshy, posterior portion of the roof of the mouth, from the palatine bones to the uvula.
Somatic	Pertaining to the nonvisceral parts of the body.
Somatomedins	A group of small polypeptides believed to be produced in the liver in response to growth hormone stimulation and to mediate the actions of growth hormone on the skeleton and other tissues.
Somatostatin	A polypeptide produced in the hypothalamus that acts to inhibit the secretion of growth hormone from the anterior pituitary. Somatostatin is also produced in the pancreatic islets, but its function there has not been established.
Somatotropic hormone	Growth hormone; an anabolic hormone secreted by the anterior pituitary that stimulates skeletal growth and protein synthesis in many organs.
Sounds of Korotkoff	The sounds heard when pressure measurements are taken. These sounds are produced by the turbulent flow of blood through an artery that has been partially constricted by a pressure cuff.
Spermatic cord	The structure of the male reproductive system composed of the ductus deferens, spermatic vessels, nerves, cremaster muscle, and connective tissue. The spermatic cord extends from a testis to the inguinal ring.
Spermatogenesis	The production of male sex gametes, or spermatozoa.
Spermatozoon	A mature male sperm cell, or gamete.
Spermiogenesis	The maturational changes that transform spermatids into spermatozoa.
Sphincter	A circular muscle that functions to constrict a body opening or the lumen of a tubular structure.
Sphincter of ampulla	The muscular constriction at the opening of the common bile and pancreatic ducts; also called the sphincter of Oddi.
Sphincter of Oddi	See sphincter of ampulla.
Sphygmomanometer	A manometer (pressure transducer) used to measure the blood pressure.
Spinal cord	The portion of the central nervous system that extends downward from the brain stem through the vertebral canal.
Spinal ganglion	A cluster of nerve cell bodies on the posterior root of a spinal nerve.
Spinal nerve	One of the 31 pairs of nerves that arise from the spinal cord.
Spindle fibers	Filaments that extend from the poles of a cell to its equator and attach to the chromosomes during the metaphase stage of cell division. Contraction of the spindle fibers pulls the chromosomes to opposite poles of the cell.
Spinous process	A sharp projection of bone or a ridge of bone, such as on the scapula.
Spiral organ	The functional unit of hearing, consisting of a basilar membrane supporting receptor hair cells and a tectorial membrane within the endolymph of the cochlear duct; also known as the organ of Corti.
Spironolactones	Diuretic drugs that act as an aldosterone antagonist.
Spleen	A large, blood-filled, glandular organ located in the upper left quadrant of the abdomen and attached by mesenteries to the stomach.
Spongy bone	Bone tissue with a lattice like structure; also called cancellous bone.
Squamous	Flat or scalelike.
Stapes	The innermost of the auditory ossicles that fits against the oval window of the inner ear; also called the stirrup.
Steroid	A lipid, derived from cholesterol, that has three 6-sided carbon rings and one 5-sided carbon ring. These form the steroid hormones of the adrenal cortex and gonads.
Stomach	A pouchlike digestive organ located between the esophagus and the duodenum.
Stratified	Arranged in layers, or strata.
Stratum basale	The deepest epidermal layer, where mitotic activity occurs.
Stratum corneum	The outer, cornified layer of the epidermis of the skin.
Stroke volume	The amount of blood ejected from each ventricle at each heartbeat.
Stroma	A connective tissue framework in an organ, gland, or other tissue.
Subarachnoid space	The space within the meninges between the arachnoid mater and pia mater, where cerebrospinal fluid flows.
Sublingual gland	One of the three pairs of salivary glands. It is located below the tongue and its duct opens to the side of the lingual frenulum.
Submandibular gland	One of the three pairs of salivary glands. It is located below the mandible and its duct opens to the side of the lingual frenulum.
Submucosa	A layer of supportive connective tissue that underlies a mucous membrane.
Submucosal plexus	A network of sympathetic and parasympathetic nerve fibers located in the submucosa tunic of the small intestine; also called the plexus of Meissner.
Substrate	In enzymatic reactions, the molecules that combine with the amino acids lining the active sites of an enzyme and are converted to products by catalysis of the enzyme.
Sulcus	A shallow impression or groove.
Superficial	Toward or near the surface.
Superficial fascia	A binding layer of connective tissue between the dermis of the skin and the underlying muscle.
Superior	Toward the upper part of a structure or toward the head; also called cephalic.
Superior vena cava	A large systemic vein that collects blood from regions of the body superior to the heart and returns it to the right atrium.
Supination	Rotation of the arm so that the palm is directed forward or anteriorly; the opposite of pronation.
Suppressor T cell	A subpopulation of T lymphocytes that acts to inhibit the production of antibodies against specific antigens by B lymphocytes.
Surface anatomy	The division of anatomy concerned with the structures that can be identified from the outside of the body.
Surfactant	A substance produced by the lungs that decreases the surface tension within the alveoli.
Suspensory ligament	1.A portion of the peritoneum that extends laterally from the surface of the ovary to the wall of the pelvic cavity. 877 2.A ligament that supports an organ or body part, such as that supporting the lens of the eye.
Sustentacular cells	Specialized cells within the testes that supply nutrients to developing spermatozoa; also called Sertoli cells or nurse cells.
Sutural bone	A small bone positioned within a suture of certain cranial bones; also called a wormian bone.
Suture	A type of fibrous joint found between bones of the skull.
Sweat gland	A skin gland that secretes a fluid substance for evaporative cooling.
Sympathetic	Pertaining to the division of the autonomic nervous system concerned with activities that, in general, arouse the body for physical activity; also called the thoracolumbar division.
Symphysis	A type of cartilaginous joint characterized by a fibrocartilaginous pad between the articulating bones, which provides slight movement.
Symphysis pubis	A slightly movable joint located anteriorly between the two pubic bones of the pelvic girdle.
Synapse	A minute space between the axon terminal of a presynaptic neuron and a dendrite of a postsynaptic neuron.
Synarthrosis	A fibrous joint, such as a syndesmosis or a suture.
Synchondrosis	A cartilaginous joint in which the articulating bones are separated by hyaline cartilage.
Syndesmosis	A type of fibrous joint in which two bones are united by an interosseous ligament.
Synergist	A muscle that assists the action of the prime mover.
Synergistic	Pertaining to regulatory processes or molecules (such as hormones) that have complementary or additive effects.
Synovial cavity	A space between the two bones of a synovial joint, filled with synovial fluid.
Synovial joint	A freely movable joint in which there is a synovial cavity between the articulating bones; also called a diarthrotic joint.
Synovial membrane	The inner membrane of a synovial capsule that secretes synovial fluid into the joint cavity.
System	A group of body organs that function together.
Systemic	Relating to the entire organism rather than to individual parts.
Systemic anatomy	The division of anatomy concerned with the structure and function of the various systems.
Systemic circulation	The portion of the circulatory system concerned with blood flow from the left ventricle of the heart to the entire body and back to the heart via the right atrium (in contrast to the pulmonary system, which involves the lungs).
Systole	The muscular contraction of a heart chamber during the cardiac cycle.
Systolic pressure	Arterial blood pressure during the ventricular systolic phase of the cardiac cycle.
Tachycardia	An excessively rapid heart rate, usually in excess of 100 beats per minute (in contrast to bradycardia, in which the heart rate is very slow).
Tactile	Pertaining to the sense of touch.
Taeniae coli	The three longitudinal bands of muscle in the wall of the large intestine.
Target organ	The specific body organ that a particular hormone affects.
Tarsal gland	An oil-secreting gland that opens on the exposed edge of each eyelid; also called a meibomian gland.
Tarsus	The region of the foot containing the seven tarsal bones.
Taste bud	An organ containing the chemocreceptors associated with the sense of taste.
T cell	A type of lymphocyte that provides cell-mediated immunity (in contrast to B lymphocytes, which provide humoral immunity through the secretion of antibodies). There are three subpopulations of T cells cytotoxic, helper, and suppressor.
Tectorial membrane	A gelatinous membrane positioned over the hair cells of the spiral organ in the cochlea.
Telencephalon	The anterior portion of the forebrain, constituting the cerebral hemispheres and related parts.
Tendo calcaneous	The tendon that attaches the calf muscles to the calcaneous bone; also called the Achilles tendon.
Tendon	A band of dense regular connective tissue that attaches muscle to bone.
Tendon sheath	A covering of synovial membrane surrounding certain tendons.
Tentorium cerebelli	An extension of dura mater that forms a partition between the cerebral hemispheres and the cerebellum and covers the cerebellum.
Teratogen	Any agent or factor that causes a physical defect in a developing embryo or fetus.
Testis	The primary reproductive organ of a male that produces spermatozoa and male sex hormones.
Testosterone	The major androgenic steroid secreted by the interstitial cells of the testes after puberty.
Tetanus	A smooth contraction of a muscle (as opposed to muscle twitching).
Thalamus	An oval mass of grey matter within the diencephalon that serves as a sensory relay centre.
Thalassemia	Any of a group of hemolytic anemias caused by the hereditary inability to produce either the alpha or beta chain of hemoglobin. It is found primarily among Mediterranean people.
Thigh	The proximal portion of the lower extremity between the hip and the knee in which the femur is located.
Third ventricle	A narrow cavity between the right and left halves of the thalamus and between the lateral ventricles that contains cerebrospinal fluid.
Thoracic	Pertaining to the chest region.
Thoracic duct	The major lymphatic vessel of the body that drains lymph from the entire body, except for the upper right quadrant, and returns it to the left subclavian vein.
Thorax	The chest.
Threshold stimulus	The weakest stimulus capable of producing an action potential in an excitable cell.
Thrombocyte	A blood platelet formed from a fragmented megakaryocyte.
Thrombus	A blood clot produced by the formation of fibrin threads around a platelet plug.
Thymus	A bilobed lymphoid organ positioned in the upper mediastinum, posterior to the sternum and between the lungs.
Thyroid cartilage	The largest cartilage in the larynx that supports and protects the vocal cords; commonly called the Adam's apple.
Thyroxine	Also called tetraiodothyronine, or T4. The major hormone secreted by the thyroid gland, which regulates the basal metabolic rate and stimulates protein synthesis in many organs. A deficiency of this hormone in early childhood produces cretinism.
Tinnitus	The spontaneous sensation of a ringing sound or other noise without sound stimuli.
Tissue	An aggregation of similar cells and their binding intercellular substance, joined to perform a specific function.
Tongue	A protrusible muscular organ on the floor of the oral cavity.
Tonsil	A node of lymphoid tissue located in the mucous membrane of the pharynx.
Toxin	A poison.
Trabeculae	A supporting framework of fibers crossing the substance of a structure, as in the lamellae of spongy bone.
Trachea	The airway leading from the larynx to the bronchi composed of cartilaginous rings and a ciliated mucosal lining of the lumen; commonly called the windpipe.
Tract	A bundle of nerve fibers within the central nervous system.
Transverse colon	A portion of the large intestine that extends from right to left across the abdomen between the hepatic and splenic flexures.
Transverse fissure	The prominent cleft that horizontally separates the cerebrum from the cerebellum.
Transverse plane	A plane that divides the body into superior and inferior portions; also called a horizontal, or cross-sectional, plane.
Tricuspid valve	The heart valve located between the right atrium and the right ventricle.
Trigone	A triangular area in the urinary bladder between the openings of the ureters and the urethra.
Triiodothyronine	Abbreviated T3; a hormone secreted in small amounts by the thyroid; the active hormone in target cells, formed from thyroxine.
Trochanter	A broad, prominent process on the proximolateral portion of the femur.
Trochlea	A pulley-like anatomical structure (e.g., the medial surface of the distal end of the humerus that articulates with the ulna).
True vocal cords	Folds of the mucous membrane in the larynx that produce sound as they are pulled taut and vibrated.
Trunk	The thorax and abdomen together.
Trypsin	A protein-digesting enzyme in pancreatic juice that is released into the small intestine.
Tubercle	A small, elevated process on a bone.
Tuberosity	An elevation or protuberance on a bone.
Tunica albuginea	A tough, fibrous tissue surrounding the testis.
Tympanic membrane	The membranous eardrum positioned between the external and middle ear.
Umbilical cord	A cordlike structure containing the umbilical arteries and vein and connecting the fetus with the placenta.
Umbilicus	The site where the umbilical cord was attached to the fetus; commonly called the navel.
Unipolar neuron	A nerve cell that has a single nerve fiber extending from its cell body.
Universal donor	A person with blood type O who is able to donate blood to people with other blood types in emergency blood transfusions.
Universal recipient	A person with blood type AB who can receive blood of any type in emergency transfusions.
Upper extremity	The appendage attached to the pectoral girdle, consisting of the shoulder, brachium, elbow, antebrachium, and hand.
Urea	The chief nitrogenous waste product of protein catabolism in the urine, formed in the liver from amino acids.
Uremia	The retention of urea and other products of protein catabolism as a result of inadequate kidney function.
Ureter	A tube that transports urine from the kidney to the urinary bladder.
Urethra	A tube that transports urine from the urinary bladder to the outside of the body.
Urinary bladder	A distensible sac that stores urine, situated in the pelvic cavity posterior to the symphysis pubis.
Urobilinogen	A compound formed from bilirubin in the small intestine; some is excreted in the feces, and some is absorbed and enters the enterohepatic circulation, where it may be excreted either in the bile or in the urine.
Uterine tube	The tube through which the ovum is transported to the uterus and the site of fertilization; also called the oviduct or fallopian tube.
Uterus	A hollow, muscular organ in which a fetus develops. It is located within the female pelvis between the urinary bladder and the rectum; commonly called the womb.
Utricle	An enlarged portion of the membranous labyrinth, located within the vestibule of the inner ear.
Uvula	A fleshy, pendulous portion of the soft palate that blocks the nasopharynx during swallowing.
Vacuole	A small space or cavity within the cytoplasm of a cell.
Vagina	A tubular organ leading from the uterus to the vestibule of the female reproductive tract that receives the male penis during coitus.
Vallate papillae	The largest papillae on the surface of the tongue. They are arranged in an inverted V-shaped pattern at the posterior portion of the tongue.
Vasectomy	Surgical removal of portions of the ductus deferentia to induce infertility.
Vasoconstriction	Narrowing of the lumen of blood vessels due to contraction of the smooth muscles in their walls.
Vasodilation	Widening of the lumen of blood vessels due to relaxation of the smooth muscles in their walls.
Vasomotor centre	A cluster of nerve cell bodies in the medulla oblongata that controls the diameter of blood vessels. It is therefore important in regulating blood pressure.
Vein	A blood vessel that conveys blood toward the heart.
Vena cava	One of two large vessels that return deoxygenated blood to the right atrium of the heart.
Ventilation	Breathing; the process of moving air into and out of the lungs.
Ventral	Toward the front or facing surface; the opposite of dorsal; also called inferior.
Ventricle	A cavity within an organ; especially those cavities in the brain that contain cerebrospinal fluid and those in the heart that contain blood to be pumped from the heart.
Venule	A small vessel that carries venous blood from capillaries to a vein.
Vermis	The coiled middle lobular structure that separates the two cerebellar hemispheres.
Vertebral canal	The tubelike cavity extending through the vertebral column that contains the spinal cord; also called the spinal canal.
Vertigo	A feeling of movement or loss of equilibrium.
Vestibular window	See oval window.
Vestibule	A space or cavity at the entrance to a canal, especially that of the nose, inner ear, or vagina.
Villus	A minute projection that extends outward into the lumen from the mucosal layer of the small intestine.
Virulent	Pathogenic; able to cause disease.
Viscera	The organs within the abdominal or thoracic cavities.
Visceral	Pertaining to the membranous covering of the viscera.
Visceral peritoneum	A serous membrane that covers the surfaces of abdominal viscera.
Visceral pleura	A serous membrane that covers the surfaces of the lungs.
Visceroceptor	A sensory receptor located within body organs that respond to information concerning the internal environment.
Vitreous humor	The transparent gel that occupies the space between the lens and retina of the eyeball.
Vulva	The external genitalia of the female that surround the opening of the vagina.
White matter	Bundles of myelinated axons located in the central nervous system.
Yellow marrow	Specialised lipid storage tissue within bone cavities.
Zygote	A fertilized egg cell formed by the union of a sperm cell and an ovum.
Zymogens	Inactive enzymes that becomes active when part of their structure is removed by the action of another enzyme or by some other means

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