saccule: A portion of the vestibular apparatus of the inner ear; contains a macula important for static equilibrium.
sagittal plane: A sectional plane that divides the body into left and right portions.
salivatory nucleus: The medullary nucleus that controls the secretory activities of the salivary glands.
salt: An inorganic compound consisting of a cation other than H+ and an anion other than OH-.
saltatory propagation: The relatively rapid propagation of an action potential between successive nodes of a myelinated axon.
sarcolemma: The cell membrane of a muscle cell.
sarcoma: A tumor of connective tissues.
sarcomere: The smallest contractile unit of a striated muscle cell.
sarcoplasm: The cytoplasm of a muscle cell.
satellite cells: See amphicytes.
scala media: See cochlear duct.
scala tympani: See tympanic duct.
scala vestibuli: See vestibular duct.
scar tissue: The thick, collagenous tissue that forms at an injury site.
Schlemm, canal of: The passageway that delivers aqueous humor from the anterior chamber of the eye to the venous circulation.
Schwann cells: Neuroglia responsible for the neurilemma that surrounds axons in the peripheral nervous system.
sciatica: Pain felt along the peripheral distribution of the sciatic nerve.
sciatic nerve: A nerve innervating the posteromedial portions of the thigh and leg.
sclera: The fibrous, outer layer of the eye that forms the white area of the anterior surface; a portion of the fibrous tunic of the eye.
sclerosis: A hardening and thickening that commonly occurs secondary to tissue inflammation.
scoliosis: An abnormal, exaggerated lateral curvature of the spine.
scrotum: The loose-fitting, fleshy pouch that encloses the testes of the male.
sebaceous glands: Glands that secrete sebum; normally associated with hair follicles.
sebum: A waxy secretion that coats the surfaces of hairs.
secondary sex characteristics: Physical characteristics that appear at puberty in response to sex hormones but are not involved in the production of gametes.
secretin: A hormone, secreted by the duodenum, that stimulates the production of buffers by the pancreas and inhibits gastric activity.
semen: The fluid ejaculate that contains spermatozoa and the secretions of accessory glands of the male reproductive tract.
semicircular ducts: The tubular components of the membranous labyrinth of the inner ear; responsible for dynamic equilibrium.
semilunar valve: A three-cusped valve guarding the exit from one of the cardiac ventricles; the pulmonary and aortic valves.
seminal vesicles: Glands of the male reproductive tract that produce roughly 60 percent of the volume of semen.
seminiferous tubules: Coiled tubules where spermatozoon production occurs in the testis.
senescence: Aging.
sensible perspiration: Water loss due to secretion by sweat glands.
septae: Partitions that subdivide an organ.
serosa: See serous membrane.
serotonin: A neurotransmitter in the central nervous system; a compound that enhances inflammation and is released by activated mast cells and basophils.
serous cell: A cell that produces a serous secretion.
serous membrane: A squamous epithelium and the underlying loose connective tissue; the lining of the pericardial, pleural, and peritoneal cavities.
serous secretion: A watery secretion that contains high concentrations of enzymes; produced by serous cells.
serum: The ground substance of blood plasma from which clotting agents have been removed.
sesamoid bone: A bone that forms within a tendon.
sigmoid colon: The S-shaped 18-cm-long portion of the colon between the descending colon and the rectum.
sign: The visible evidence of the presence of a disease.
simple epithelium: An epithelium containing a single layer of cells above the basement membrane.
sinoatrial (SA) node: The natural pacemaker of the heart; situated in the wall of the right atrium.
sinus: A chamber or hollow in a tissue; a large, dilated vein.
sinusitis: An inflammation of a nasal sinus.
sinusoid: An extensive network of vessels in the liver, adrenal cortex, spleen, and pancreas; similar in histological structure to capillaries.
skeletal muscle: A contractile organ of the muscular system.
skeletal muscle tissue: A contractile tissue dominated by skeletal muscle fibers; characterized as striated, voluntary muscle.
sliding filament theory: The concept that a sarcomere shortens as the thick and thin filaments slide past one another.
small intestine: The duodenum, jejunum, and ileum; the digestive tract between the stomach and the large intestine.
smegma: The secretion of the preputial glands of the penis or clitoris.
smooth endoplasmic reticulum (SER): A membranous organelle in which lipid and carbohydrate synthesis and storage occur.
smooth muscle tissue: Muscle tissue in the walls of many visceral organs; characterized as nonstriated, involuntary muscle.
soft palate: The fleshy posterior extension of the hard palate, separating the nasopharynx from the oral cavity.
sole: The inferior surface of the foot.
solute: Any materials dissolved in a solution.
solution: A fluid containing dissolved materials.
solvent: The fluid component of a solution.
soma: Body; the body of a neuron; also called cell body.
somatic: Pertaining to the body.
somatic nervous system (SNS): The efferent division of the nervous system that innervates skeletal muscles.
somatomedins: Compounds stimulating tissue growth; released by the liver after the secretion of growth hormone; also called insulin-like growth factors.
somatotropin: Growth hormone; produced by the anterior pituitary in response to growth hormone–releasing hormone (GH-RH).
sperm: See spermatozoon.
spermatic cord: Collectively, the spermatic vessels, nerves, lymphatic vessels, and the ductus deferens, extending between the testes and the proximal end of the inguinal canal.
spermatids: The product of meiosis in males; cells that differentiate into spermatozoa.
spermatocyte: A cell of the seminiferous tubules that is engaged in meiosis.
spermatogenesis: Spermatozoon production.
spermatogonia: Stem cells whose mitotic divisions give rise to other stem cells and to primary spermatocytes.
spermatozoon/spermatozoa: A male gamete; also called sperm.
spermicide: A compound toxic to spermatozoa; used as a contraceptive method.
spermiogenesis: The process of spermatid differentiation that leads to the formation of physically mature spermatozoa.
sphincter: A muscular ring that contracts to close the entrance or exit of an internal passageway.
spinal nerve: One of 31 pairs of nerves that originate on the spinal cord from anterior and posterior roots.
spindle apparatus: A muscle spindle (intrafusal fibers) and its sensory and motor innervation.
spinocerebellar tracts: Ascending tracts that carry sensory information to the cerebellum.
spinothalamic tracts: Ascending tracts that carry poorly localized touch, pressure, pain, vibration, and temperature sensations to the thalamus.
spinous process: The prominent posterior projection of a vertebra; formed by the fusion of two laminae.
spleen: A lymphoid organ important for the phagocytosis of red blood cells, the immune response, and lymphocyte production.
splenectomy: The surgical removal of the spleen.
sprain: A forceful distortion of an articulation that produces damage to the capsule, ligaments, or tendons but not dislocation.
sputum: A viscous mucus that is transported to the pharynx by the mucus escalator of the respiratory tract and is ejected from the mouth.
squama: A broad, flat surface.
squamous: Flattened.
squamous epithelium: An epithelium whose superficial cells are flattened and platelike.
stapedius: A muscle of the middle ear whose contraction tenses the auditory ossicles and reduces the forces transmitted to the oval window.
stapes: The auditory ossicle attached to the tympanic membrane.
statoconia: Densely packed calcium carbonate crystals forming masses that are attached, by a gelatinous matrix, to hair cell receptors of maculae in the vestibule of the inner ear.
stenosis: A constriction or narrowing of a passageway.
stereocilia: Elongate microvilli characteristic of the epithelium of the epididymis, portions of the ductus deferens, and the inner ear.
steroid: A ring-shaped lipid structurally related to cholesterol.
stimulus: An environmental change that produces a change in cellular activities; often used to refer to events that alter the transmembrane potentials of excitable cells.
stratified: Containing several layers.
stratum: A layer.
stretch receptors: Sensory receptors that respond to stretching of the surrounding tissues.
stroma: The connective tissue framework of an organ; distinguished from the functional cells (parenchyma) of that organ.
subarachnoid space: A meningeal space containing cerebrospinal fluid; the area between the arachnoid membrane and the pia mater.
subclavian: Pertaining to the region immediately posterior and inferior to the clavicle.
subcutaneous layer: The layer of loose connective tissue below the dermis; also called hypodermis or superficial fascia.
sublingual salivary glands: Mucus-secreting salivary glands inferior to the tongue.
submandibular salivary glands: Salivary glands nestled in depressions on the medial surfaces of the mandible; salivary glands that produce a mixture of mucins and enzymes (salivary amylase).
submucosa: The region between the muscularis mucosae and the muscularis externa.
submucosal glands: Mucous glands in the submucosa of the duodenum; also called Brunner’s glands.
subserous fascia: The loose connective tissue layer beneath the serous membrane that lines the ventral body cavity.
substrate: A participant (product or reactant) in an enzyme-catalyzed reaction.
sulcus: A groove or furrow.
summation: The temporal or spatial addition of stimuli.
superficial fascia: See subcutaneous layer.
superior: Above, in reference to a portion of the body in the anatomical position.
superior vena cava (SVC): The vein that carries blood to the right atrium from parts of the body that are superior to the heart.
supination: The rotation of the forearm such that the palm faces anteriorly.
supine: Lying face up, with palms facing anteriorly.
suppressor T cells: Lymphocytes that inhibit B cell activation and the secretion of antibodies by plasma cells.
suprarenal gland: See adrenal gland.
surfactant: A lipid secretion that coats the alveolar surfaces of the lungs and prevents their collapse.
sustentacular cells: Supporting cells of the seminiferous tubules of the testis; responsible for the differentiation of spermatids, the maintenance of the blood–testis barrier, and the secretion of inhibin, androgen-binding protein, and Müllerian-inhibiting factor.
sutural bones: Irregular bones that form in fibrous tissue between the flat bones of the developing cranium; also called Wormian bones.
suture: A fibrous joint between flat bones of the skull.
sympathectomy: The transection of the sympathetic innervation to a region.
sympathetic division: The division of the autonomic nervous system that is responsible for "fight or flight" reactions; primarily concerned with the elevation of metabolic rate and increased alertness.
sympathomimetic drugs: Drugs that mimic the actions of sympathetic stimulation.
symphysis: A fibrous amphiarthrosis, such as that between adjacent vertebrae or between the pubic bones of the ossa coxae.
symptom: An abnormality of function as a result of disease.
synapse: The site of communication between a nerve cell and some other cell; if the other cell is not a neuron, the term neuromuscular or neuroglandular junction is often used.
synaptic delay: The period between the arrival of an impulse at the presynaptic membrane and the initiation of an action potential in the postsynaptic membrane.
synarthrosis: A joint that does not permit relative movement between the articulating elements; see lambdoidal suture.
synchondrosis: A cartilaginous synarthrosis, such as the articulation between the epiphysis and diaphysis of a growing bone.
syncope: A sudden, transient loss of consciousness; a faint.
syncytial trophoblast: The multinucleate cytoplasmic layer that covers the blastocyst; responsible for uterine erosion and implantation.
syncytium: A multinucleate mass of cytoplasm, produced by the fusion of cells or repeated mitoses without cytokinesis.
syndesmosis: A fibrous amphiarthrosis.
syndrome: A discrete set of symptoms that occur together.
syneresis: Clot retraction.
synergist: A muscle that assists a prime mover in performing its primary action.
synostosis: A synarthrosis formed by the fusion of the articulating elements.
synovial cavity: A fluid-filled chamber in a synovial joint.
synovial fluid: The substance secreted by synovial membranes that lubricates joints.
synovial joint: A freely movable joint where the opposing bone surfaces are separated by synovial fluid; a diarthrosis.
synovial membrane: An incomplete layer of fibroblasts confronting the synovial cavity, plus the underlying loose connective tissue.
synthesis: Manufacture; anabolism.
system: An interacting group of organs that performs one or more specific functions.
systemic circuit: The vessels between the aortic semilunar valve and the entrance to the right atrium; the circulatory system other than the vessels of the pulmonary circuit.
systole: The period of cardiac contraction.
systolic pressure: The peak arterial pressure measured during ventricular systole.