calcanean tendon: The large tendon that inserts on the calcaneus; tension on this tendon produces plantar flexion of the foot; also called Achilles tendon.

calcaneus: The heelbone, the largest of the tarsal bones.

calcification: The deposition of calcium salts within a tissue.

calcitonin: The hormone secreted by C cells of the thyroid when calcium ion concentrations are abnormally high; restores homeostasis by increasing the rate of bone deposition and the rate of calcium loss at the kidneys.

calculus/calculi: Concretions of insoluble materials that form within body fluids, especially the gallbladder, kidneys, or urinary bladder.

callus: A localized thickening of the epidermis due to chronic mechanical stresses; a thickened area that forms at the site of a bone break as part of the repair process.

calorie (c): The amount of heat that is required to raise the temperature of 1 gram of water 1°C.

Calorie (C): The amount of heat that is required to raise the temperature of 1 kilogram of water 1°C; also called kilocalorie.

calorigenic effect: The stimulation of energy production and heat loss by thyroid hormones.

calvaria: The skullcap, consisting of the superior portions of the frontal, parietal, and occipital bones.

calyx/calyces: Cup-shaped divisions of the renal pelvis.

canaliculi: Microscopic passageways between cells; bile canaliculi carry bile to bile ducts in the liver; in bone, canaliculi permit the diffusion of nutrients and wastes to and from osteocytes.

cancellous bone: Spongy bone, composed of a network of bony struts.

cancer: A malignant tumor that can undergo metastasis.

cannula: A tube that can be inserted into the body; commonly placed in blood vessels prior to transfusion or dialysis.

canthus, medial and lateral: The angles formed at either corner of the eye between the upper and lower eyelids.

capacitation: The activation process that must occur before a spermatozoon can successfully fertilize an oocyte; occurs in the vagina after ejaculation.

capillary: A small blood vessel, located between an arteriole and a venule, whose thin wall permits the diffusion of gases, nutrients, and wastes between plasma and interstitial fluids.

capitulum: A general term for a small, elevated articular process; refers to the rounded distal surface of the humerus that articulates with the head of the radius.

caput: The head.

carbaminohemoglobin: Hemoglobin bound to carbon dioxide molecules.

carbohydrase: An enzyme that breaks down carbohydrate molecules.

carbohydrate: An organic compound containing carbon, hydrogen, and oxygen in a ratio that approximates 1:2:1.

carbon dioxide: CO₂; a compound produced by the decarboxylation reactions of aerobic metabolism.

carbonic anhydrase: An enzyme that catalyzes the reaction H₂O + CO₂ H₂CO₃; important in carbon dioxide transport, gastric acid secretion, and renal pH regulation.

carboxypeptidase: A protease that breaks down proteins and releases amino acids.

carcinogenic: Stimulating cancer formation in affected tissues.

cardia: The area of the stomach surrounding its connection with the esophagus.

cardiac: Pertaining to the heart.

cardiac cycle: One complete heartbeat, including atrial and ventricular systole and diastole.

cardiac glands: Mucous glands characteristic of the cardia of the stomach.

cardiac output: The amount of blood ejected by the left ventricle each minute; normally about 5 liters.

cardiac reserve: The potential percentage increase in cardiac output above resting levels.

cardiac tamponade: A compression of the heart due to fluid accumulation in the pericardial cavity.

cardiocyte: A cardiac muscle cell.

cardiomyopathy: A progressive disease characterized by damage to the cardiac muscle tissue.

cardiopulmonary resuscitation (CPR): A method of artificially maintaining respiratory and circulatory functions.

cardiovascular: Pertaining to the heart, blood, and blood vessels.

cardiovascular centers: Poorly localized centers in the reticular formation of the medulla oblongata of the brain; includes cardioacceleratory, cardioinhibitory, and vasomotor centers.

cardium: The heart.

carina: A ridge on the inner surface of the base of the trachea that runs anteroposteriorly, between two primary bronchi.

carotene: A yellow-orange pigment, found in carrots and in green and orange leafy vegetables, that the body can convert to vitamin A.

carotid artery: The principal artery of the neck, servicing cervical and cranial structures; one branch, the internal carotid, provides a major blood supply to the brain.

carotid body: A group of receptors, adjacent to the carotid sinus, that are sensitive to changes in the carbon dioxide levels, pH, and oxygen concentrations of arterial blood.

carotid sinus: A dilated segment at the base of the internal carotid artery whose walls contain baroreceptors sensitive to changes in blood pressure.

carotid sinus reflex: Reflexive changes in blood pressure that maintain homeostatic pressures at the carotid sinus, stabilizing blood flow to the brain.

carpus/carpal: The wrist.

cartilage: A connective tissue with a gelatinous matrix that contains an abundance of fibers.

castration: The removal of the testes; also called bilateral orchiectomy.

catabolism: The breakdown of complex organic molecules into simpler components, accompanied by the release of energy.

catalyst: A substance that accelerates a specific chemical reaction but that is not altered by the reaction.

cataract: A reduction in lens transparency that causes visual impairment.

catecholamine: Epinephrine, norepinephrine, dopamine, and related compounds.

cathepsins: Enzymes, present in the sarcoplasm of skeletal muscle cells, that can break down contractile proteins, thereby providing amino acids that can act as a supplemental energy source.

catheter: A tube surgically inserted into a body cavity or along a blood vessel or excretory passageway for the collection of body fluids, monitoring of blood pressure, or introduction of medications or radiographic dyes.

cation: An ion that bears a positive charge.

cauda equina: Spinal nerve roots distal to the tip of the adult spinal cord; they extend caudally inside the vertebral canal en route to lumbar and sacral segments.

caudal/caudally: Closest to or toward the tail (coccyx).

caudate nucleus: One of the cerebral nuclei of the extrapyramidal system; involved with the unconscious control of muscular activity.

cavernous tissue: Erectile tissue that can be engorged with blood; located in the penis (males) and clitoris (females).

cecum: An expanded pouch at the start of the large intestine.

cell: The smallest living unit in the human body.

cell body: Body; the body of a neuron; also called soma.

cell-mediated immunity: Resistance to disease through the activities of sensitized T cells that destroy antigen-bearing cells by direct contact or through the release of lymphotoxins; also called cellular immunity.

cellulitis: Diffuse inflammation, normally involving areas of loose connective tissue, such as the subcutaneous layer.

cementum: Bony material that covers the root of a tooth and is not shielded by a layer of enamel.

center of ossification: The site in a connective tissue where bone formation begins.

central canal: Longitudinal canal in the center of an osteon that contains blood vessels and nerves; also called Haversian canal; a passageway along the longitudinal axis of the spinal cord that contains cerebrospinal fluid.

central nervous system (CNS): The brain and spinal cord.

central sulcus: A groove in the surface of a cerebral hemisphere, between the primary sensory and primary motor areas of the cortex.

centriole: A cylindrical intracellular organelle composed of nine groups of microtubules, three in each group; functions in mitosis or meiosis by organizing the microtubules of the spindle apparatus.

centromere: The localized region where two chromatids remain connected after the chromosomes have replicated; site of spindle fiber attachment.

centrosome: A region of cytoplasm that contains a pair of centrioles oriented at right angles to one another.

centrum: The body of a vertebra.

cephalic: Pertaining to the head.

cerebellum: The posterior portion of the metencephalon, containing the cerebellar hemispheres; includes the arbor vitae, cerebellar nuclei, and cerebellar cortex.

cerebral cortex: An extensive area of neural cortex covering the surfaces of the cerebral hemispheres.

cerebral hemispheres: A pair of expanded portions of the cerebrum covered in neural cortex.

cerebral nuclei: Nuclei of the cerebrum that are important in the subconscious control of skeletal muscle activity.

cerebral palsy: A chronic condition resulting from damage to motor areas of the brain during development or at delivery.

cerebral peduncle: A mass of nerve fibers on the ventrolateral surface of the mesencephalon; contains ascending tracts that terminate in the thalamus and descending tracts that originate in the cerebral hemispheres.

cerebrospinal fluid (CSF): Fluid bathing the internal and external surfaces of the central nervous system; secreted by the choroid plexus.

cerebrovascular accident (CVA): The occlusion of a blood vessel that supplies a portion of the brain, resulting in damage to the dependent neurons; also called stroke.

cerebrum: The largest portion of the brain, composed of the cerebral hemispheres; includes the cerebral cortex, the cerebral nuclei, and the internal capsule.

cerumen: The waxy secretion of the ceruminous glands along the external auditory canal.

ceruminous glands: Integumentary glands that secrete cerumen.

cervical enlargement: A thickening of the cervical portion of the spinal cord caused by the abundance of central nervous system neurons involved with motor control of the upper limbs.

cervix: The inferior portion of the uterus.

cesarean section: The surgical delivery of an infant via an incision through the lower abdominal wall and uterus.

chalazion: An inflammation and distension of a Meibomian gland on the eyelid; also called sty.

chancre: A skin lesion that develops at the primary site of a syphilis infection.

chemoreception: The detection of changes in the concentrations of dissolved compounds or gases.

chemotaxis: The attraction of phagocytic cells to the source of abnormal chemicals in tissue fluids.

chemotherapy: The treatment of illness through the administration of specific chemicals.

chloride shift: The movement of plasma chloride ions into red blood cells in exchange for bicarbonate ions generated by the intracellular dissociation of carbonic acid.

cholecystitis: An inflammation of the gallbladder.

cholecystokinin (CCK): A duodenal hormone that stimulates the contraction of the gallbladder and the secretion of enzymes by the exocrine pancreas; also called pancreozymin.

cholelithiasis: The formation or presence of gallstones.

cholesterol: A steroid component of cell membranes and a substrate for the synthesis of steroid hormones and bile salts.

choline: A breakdown product or precursor of acetylcholine.

cholinergic synapse: A synapse where the presynaptic membrane releases acetylcholine on stimulation.

cholinesterase: The enzyme that breaks down and inactivates acetylcholine.

chondrocyte: A cartilage cell.

chondroitin sulfate: The predominant proteoglycan in cartilage, responsible for the gelatinous consistency of the matrix.

chordae tendineae: Fibrous cords that stabilize the position of the AV valves in the heart, preventing backflow during ventricular systole.

chorion/chorionic: An extraembryonic membrane, consisting of the trophoblast and underlying mesoderm, that forms the placenta.

choroid: The middle, vascular layer in the wall of the eye.

choroid plexus: The vascular complex in the roof of the third and fourth ventricles of the brain, responsible for the production of cerebrospinal fluid.

chromatid: One complete copy of a DNA strand and its associated nucleoproteins.

chromatin: A histological term referring to the grainy material visible in cell nuclei during interphase; the appearance of the DNA content of the nucleus when the chromosomes are uncoiled.

chromosomes: Dense structures, composed of tightly coiled DNA strands and associated histones, that become visible in the nucleus when a cell prepares to undergo mitosis or meiosis; normal human somatic cells contain 46 chromosomes apiece.

chronic: Habitual or long term.

chylomicrons: Relatively large droplets that may contain triglycerides, phospholipids, and cholesterol in association with proteins; synthesized and released by intestinal cells and transported to the venous blood by the lymphatic system.

chyme: A semifluid, acidic mixture of ingested food and digestive secretions that forms in the stomach during the early phases of digestion.

chymotrypsin: A protease in the small intestine.

chymotrypsinogen: The inactive proenzyme, secreted by the pancreas, that is subsequently converted to chymotrypsin.

ciliary body: A thickened region of the choroid that encircles the lens of the eye; includes the ciliary muscle and the ciliary processes that support the suspensory ligaments of the lens.

cilium/cilia: A slender organelle that extends above the free surface of an epithelial cell and generally undergoes cycles of movement; composed of a basal body and microtubules in a 9 + 2 array.

circulatory system: The network of blood vessels and lymphatic vessels that facilitate the distribution and circulation of extracellular fluid.

circumduction: A movement at a synovial joint in which the distal end of the bone describes a circle but the shaft does not rotate.

circumvallate papilla: One of the large, dome-shaped papillae on the superior surface of the tongue that form a V, separating the body of the tongue from the root.

cirrhosis: A liver disorder characterized by the degeneration of hepatocytes and their replacement by fibrous connective tissue.

cisterna: An expanded chamber.

citric acid cycle: See TCA cycle.

cleavage: Mitotic divisions that follow the fertilization of an ovum and lead to the formation of a blastocyst.

cleavage lines: Stress lines in the skin that follow the orientation of major bundles of collagen fibers in the dermis.

clitoris: A small erectile organ of females that is the developmental equivalent of the penis in males.

clone: As a verb, the production of genetically identical cells; as a noun, a genetically identical copy.

clonus: Rapid cycles of muscle contraction and relaxation.

clot: A network of fibrin fibers and trapped blood cells; also called a thrombus if it occurs within the circulatory system.

clotting factors: Plasma proteins, synthesized by the liver, that are essential to the clotting response.

clotting response: The series of events that result in the formation of a clot.

coccygeal ligament: The fibrous extension of the dura mater and filum terminale; provides longitudinal stabilization to the spinal cord.

coccyx: The terminal portion of the spinal column, consisting of relatively tiny, fused vertebrae.

cochlea: The spiral portion of the bony labyrinth of the inner ear that surrounds the organ of hearing.

cochlear duct: The central membranous tube within the cochlea that is filled with endolymph and contains the organ of Corti; also called scala media.

codon: A sequence of three nitrogenous bases along an mRNA strand that will specify the location of a single amino acid in a peptide chain.

coelom: The ventral body cavity, lined by a serous membrane and subdivided during fetal development into the pleural, pericardial, and abdominopelvic (peritoneal) cavities.

coenzymes: Complex organic cofactors; most are structurally related to vitamins.

cofactor: Ions or molecules that must be attached to the active site before an enzyme can function; examples include mineral ions and several vitamins.

colectomy: The surgical removal of part or all of the colon.

colitis: An inflammation of the colon.

collagen: A strong, insoluble protein fiber common in connective tissues.

collateral ganglion: A sympathetic ganglion situated anterior to the spinal column and separate from the sympathetic chain.

Colles' fracture: A fracture of the distal end of the radius and possibly the ulna, with posterior and dorsal displacement of the distal bone fragments.

colliculus/colliculi: A little mound; in the brain, refers to one of the thickenings in the roof of the mesencephalon; the superior colliculus is associated with the visual system, and the inferior colliculi with the auditory system.

colloid/colloidal suspension: A solution containing large organic molecules in suspension.

colon: The large intestine.

colonoscope: A fiber-optic device for examining the interior of the colon.

colostomy: The surgical connection of a portion of the colon to the body wall, sometimes performed after a colectomy to permit the discharge of fecal materials.

colostrum: The secretion of the mammary glands at the time of childbirth and for a few days thereafter; contains more protein and less fat than does the milk secreted later.

coma: An unconscious state from which an individual cannot be aroused, even by strong stimuli.

comedo: An inflamed sebaceous gland.

comminuted: Broken or crushed into small pieces.

commissure: A crossing over from one side to another.

common bile duct: The duct formed by the union of the cystic duct from the gallbladder and the bile ducts from the liver; terminates at the duodenal ampulla, where it meets the pancreatic duct.

common pathway: In the clotting response, the events that begin with the appearance of thromboplastin and end with the formation of a clot.

compact bone: Dense bone that contains parallel osteons.

compensation curves: The cervical and lumbar curves that develop to center the body weight over the legs.

complement: A system of 11 plasma proteins that interact in a chain-reaction after exposure to activated antibodies or the surfaces of certain pathogens; complement proteins promote cell lysis, phagocytosis, and other defense mechanisms.

compliance: Distensibility; the ability of certain organs to tolerate changes in volume; indicates the presence of elastic fibers and smooth muscles.

compound: A molecule containing two or more elements in combination.

concentration: The amount (in grams) or number of atoms, ions, or molecules (in moles) per unit volume.

concentration gradient: Regional differences in the concentration of a particular substance.

conception: Fertilization.

concha/conchae: Three pairs of thin, scroll-like bones that project into the nasal cavities; the superior and medial conchae are part of the ethmoid bone, and the inferior conchae are separate bones.

concussion: A violent blow or shock; loss of consciousness due to a violent blow to the head.

condyle: A rounded articular projection on the surface of a bone.

cone: A photoreceptor of the retina, responsible for color vision.

congenital: Present at birth.

congestive heart failure (CHF): The failure to maintain adequate cardiac output due to circulatory problems or myocardial damage.

conjunctiva: A layer of stratified squamous epithelium that covers the inner surfaces of the eyelids and the anterior surface of the eye to the edges of the cornea.

conjunctivitis: An inflammation of the conjunctiva.

connective tissue: One of the four primary tissue types; provides a structural framework that stabilizes the relative positions of the other tissue types; includes connective tissue proper, cartilage, bone, and blood; contains cell products, cells, and ground substance.

continuous propagation: The propagation of an action potential along an unmyelinated axon or a muscle cell membrane, wherein the action potential affects every portion of the membrane surface.

contractility: The ability to contract; possessed by skeletal, smooth, and cardiac muscle cells.

contralateral reflex: A reflex that affects the opposite side of the body from the stimulus.

conus medullaris: The conical tip of the spinal cord that gives rise to the filum terminale.

convergence: In the nervous system, the innervation of a single neuron by axons from several neurons; most common along motor pathways.

coracoid process: A hook-shaped process of the scapula that projects above the anterior surface of the capsule of the shoulder joint.

Cori cycle: The metabolic exchange of lactic acid from skeletal muscle for glucose from the liver; performed during the recovery period after muscular exertion.

cornea: The transparent portion of the fibrous tunic of the anterior surface of the eye.

corniculate cartilages: A pair of small laryngeal cartilages.

cornu: Horn-shaped.

corona radiata: A layer of follicle cells surrounding a secondary oocyte at ovulation.

coronoid: Hooked or curved.

corpora quadrigemina: The superior and inferior colliculi of the mesencephalic tectum (roof) in the brain.

corpus/corpora: Body.

corpus callosum: The bundle of axons that links centers in the left and right cerebral hemispheres.

corpora cavernosa: Two parallel masses of erectile tissue within the body of the penis (males) or clitoris (females).

corpus luteum: The progestin-secreting mass of follicle cells that develops in the ovary after ovulation.

corpus spongiosum: The mass of erectile tissue that surrounds the urethra in the penis and expands distally to form the glans.

cortex: The outer layer or portion of an organ.

Corti, organ of: The receptor complex in the cochlear duct that includes the inner and outer hair cells, supporting cells and structures, and the tectorial membrane; provides the sensation of hearing.

corticobulbar tracts: Descending tracts that carry information or commands from the cerebral cortex to nuclei and centers in the brain stem.

corticospinal tracts: Descending tracts that carry motor commands from the cerebral cortex to the anterior gray horns of the spinal cord.

corticosteroid: A steroid hormone produced by the adrenal cortex.

corticosterone: A corticosteroid secreted by the zona fasciculata of the adrenal cortex; a glucocorticoid.

corticotropin-releasing hormone (CRH): The releasing hormone, secreted by the hypothalamus, that stimulates secretion of adrenocorticotropic hormone by the anterior pituitary.

cortisol: A corticosteroid secreted by the zona fasciculata of the adrenal cortex; a glucocorticoid.

costa/costae: A rib.

cotransport: The membrane transport of a nutrient, such as glucose, in company with the movement of an ion, normally sodium; transport requires a carrier protein but does not involve direct ATP expenditure and can occur regardless of the concentration gradient for the nutrient.

countercurrent exchange: The diffusion between two solutions that travel in opposite directions.

countercurrent multiplication: Active transport between two limbs of a loop that contains a fluid moving in one direction; responsible for the concentration of urine in the kidney tubules.

covalent bond: A chemical bond between atoms that involves the sharing of electrons.

cranial: Pertaining to the head.

cranial nerves: Peripheral nerves originating at the brain.

craniosacral division: See parasympathetic division.

craniostenosis: A skull deformity caused by the premature closure of the cranial sutures.

cranium: The braincase; the skull bones that surround and protect the brain.

creatine: A nitrogenous compound, synthesized in the body, that can form a high-energy bond by connecting to a phosphate group and that serves as an energy reserve.

creatine phosphate: A high-energy compound in muscle cells; during muscle activity, the phosphate group is donated to ADP, regenerating ATP; also called phosphorylcreatine.

creatinine: A breakdown product of creatine metabolism.

crenation: Cellular shrinkage due to an osmotic movement of water out of the cytoplasm.

cribriform plate: A portion of the ethmoid bone that contains the foramina used by the axons of olfactory receptors en route to the olfactory bulbs of the cerebrum.

cricoid cartilage: A ring-shaped cartilage that forms the inferior margin of the larynx.

crista/cristae: A ridge-shaped collection of hair cells in the ampulla of a semicircular canal; the crista and cupula form a receptor complex sensitive to movement along the plane of the canal.

cross-bridge: A myosin head that projects from the surface of a thick filament and that can bind to an active site of a thin filament in the presence of calcium ions.

cruciate ligaments: A pair of intracapsular ligaments (anterior and posterior) in the knee.

cryosurgery: A surgical technique that involves freezing and killing cells in a localized area.

cryptorchid testis: An undescended testis that is in the abdominopelvic cavity rather than in the scrotum.

cuneiform cartilages: A pair of small cartilages in the larynx.

cupula: A gelatinous mass that is located in the ampulla of a semicircular canal in the inner ear and whose movement stimulates the hair cells of the crista.

curare: A toxin that prevents neural stimulation of neuromuscular junctions.

Cushing's disease: A condition caused by the oversecretion of adrenal steroids.

cutaneous membrane: The epidermis and papillary layer of the dermis.

cuticle: The layer of dead, keratinized cells that surrounds the shaft of a hair; for nails, see eponychium.

cyanosis: A bluish coloration of the skin due to the presence of deoxygenated blood in vessels near the body surface.

cyst: A fibrous capsule containing fluid or other material.

cystic duct: A duct that carries bile between the gallbladder and the common bile duct.

cystitis: An inflammation of the urinary bladder.

cytochrome: A pigment component of the electron transport system; a structural relative of heme.

cytokinesis: The cytoplasmic movement that separates two daughter cells at the completion of mitosis.

cytology: The study of cells.

cytoplasm: The material between the cell membrane and the nuclear membrane; cell contents.

cytosine: A pyrimidine; one of the nitrogenous bases in the nucleic acids RNA and DNA.

cytoskeleton: A network of microtubules and microfilaments in the cytoplasm.

cytosol: The fluid portion of the cytoplasm.

cytotoxic: Poisonous to cells.

cytotoxic T cells: Lymphocytes involved in cell-mediated immunity that kill target cells by direct contact or by the secretion of lymphotoxins; also called killer T cells and TC cells.