Tetany (TET-a-ne) Hyperexcitability of neurons and muscle fibers
(cells) caused by hypocalcemia and characterized by intermit-
tent or continuous tonic muscular contractions; may be due to
hypoparathyroidism.
Thalamus (THAL-a-mus) A large, oval structure located bilaterally
on either side of the third ventricle, consisting of two masses of
gray matter organized into nuclei; main relay center for sensory
impulses ascending to the cerebral cortex.
Thermoreceptor (THER-mo-re-sep-tor) Sensory receptor that detects
changes in temperature.
Thigh The portion of the lower limb between the hip and the knee.
Third ventricle (VEN-tri-kul) A slitlike cavity between the right and left
halves of the thalamus and between the lateral ventricles of the
brain.
Thirst center A cluster of neurons in the hypothalamus that is
sensitive to the osmotic pressure of extracellular fluid and brings
about the sensation of thirst.
Thoracic cavity (tho-RAS-ik) A cavity that contains two pleural cavi-
ties, the mediastinum, and the pericardial cavity.
Thoracic duct A lymphatic vessel that begins as a dilation called the
cisterna chyli, receives lymph from the left side of the head, neck,
and chest, the left arm, and the entire body below the ribs, and
empties into the left subclavian vein. Also called the left lymphat-
ic duct (lim-FAT-ik).
Thoracolumbar outflow (tho'-ra-ko-LUM-bar) The axons of sympa-
thetic preganglionic neurons, which have their cell bodies in the
lateral gray columns of the thoracic segments and first two or
three lumbar segments of the spinal cord.
Thorax (THO-raks) The chest.
Threshold potential The membrane voltage that must be reached to
trigger an action potential.
Threshold stimulus Any stimulus strong enough to initiate an action
potential or activate a sensory receptor.
Thrombin (THROM-bin) The active enzyme formed from prothrombin
that converts fibrinogen to fibrin during the formation of a blood clot.
Thrombolytic agent (throm-bo-LIT-ik) Chemical substance injected
into the body to dissolve blood clots and restore circulation;
mechanism of action is direct or indirect activation of plasmino-
gen; examples include tissue plasminogen activator (t-PA), strep-
tokinase, and urokinase.
Thrombosis (throm-BO-sis) The formation of a clot in an unbroken
blood vessel, usually a vein.
Thrombus (THROM-bus) A stationary clot formed in an unbroken
blood vessel, usually a vein.
Thymus (THf-mus) A bilobed organ, located in the superior mediasti-
num posterior to the sternum and between the lungs, in which T
cells develop immunocompetence.
Thyroglobulin (TGB) (thT-ro-GLO-bu-lin) A large glycoprotein molecule
produced by follicular cells of the thyroid gland in which some
tyrosines are iodinated and coupled to form thyroid hormones.
Thyroid cartilage (THT-royd KAR-ti-lij) The largest single cartilage of
the larynx, consisting of two fused plates that form the anterior
wall of the larynx.
Thyroid follicle (FOL-i-kul) Spherical sac that forms the parenchyma
of the thyroid gland and consists of follicular cells that produce
thyroxine (T4) and triiodothyronine (T3).
Thyroid gland An endocrine gland with right and left lateral lobes on
either side of the trachea connected by an isthmus; located ante-
rior to the trachea just inferior to the cricoid cartilage; secretes
thyroxine (T4), triiodothyronine (T3), and calcitonin (CT).
Thyroid-stimulating hormone (TSH) A hormone secreted by the
anterior pituitary that stimulates the synthesis and secretion of
thyroxine (T4) and triiodothyronine (T3).
Thyroxine (T4) (thT-ROK-sen) A hormone secreted by the thyroid
gland that regulates metabolism, growth and development, and
the activity of the nervous system.
Tic Spasmodic, involuntary twitching of muscles that are normally
under voluntary control.
Tidal volume The volume of air breathed in and out in any one
breath; about 500 mL in quiet, resting conditions.
Tissue A group of similar cells and their intercellular substance
joined together to perform a specific function.
Tissue factor (TF) A factor, or collection of factors, whose appear-
ance initiates the blood clotting process. Also called thromboplas-
tin (throm-bo-PLAS-tin).
Tissue plasminogen activator (t-PA) (plaz-MIN-o-gen) An enzyme that
dissolves small blood clots by initiating a process that converts
plasminogen to plasmin, which degrades the fibrin of a clot.
Tongue A large skeletal muscle covered by a mucous membrane
located on the floor of the oral cavity.
Tonicity (to-NIS-i-te) A measure of the concentration of impermeable
solute particles in a solution relative to cytosol. When cells are
bathed in an isotonic solution, they neither shrink nor swell.
Tonsil (TON-sil) An aggregation of large lymphatic nodules embed-
ded in the mucous membrane of the throat.
Torn cartilage A tearing of an articular disc (meniscus) in the knee.
Total lung capacity The sum of tidal volume, inspiratory reserve
volume, expiratory reserve volume, and residual volume; about
6000 mL in males.
Trabecula (tra-BEK-D-la) Irregular latticework of thin plates of spongy
bone. Fibrous cord of connective tissue serving as supporting
fiber by forming a septum extending into an organ from its wall or
capsule.
P lu r a l
is trabeculae (tra-BEK-u-le).
Trachea (TRA-ke-a) Tubular air passageway extending from the lar-
ynx to the fifth thoracic vertebra. Also called the windpipe.
Tract A bundle of nerve axons in the central nervous system.
Transcription (trans-KRIP-shun) The first step in the expression of
genetic information in which a single strand of DNA serves as a
template for the formation of an RNA molecule.
Translation (trans-LA-shun) The synthesis of a new protein on a ribo-
some as dictated by the sequence of codons in messenger RNA.
Transverse colon (trans-VERS KO-lon) The portion of the large intes-
tine extending across the abdomen from the right colic (hepatic)
flexure to the left colic (splenic) flexure.
Transverse fissure (FISH-er) The deep cleft that separates the cere-
brum from the cerebellum.
Glossary 551
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