| third law of thermodynamics | <chemistry> The entropy of a perfect crystal at 0 K is zero. (09 Jan 1998) |
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| third molar | Eighth permanent tooth in the maxilla and mandible on each side, making it the most posterior tooth in human dentition; usually erupts between the seventeenth and twenty-third years; the roots are often fused, the separation being marked only by grooves; because it tends to erupt in an anterosuperior direction, the lower third molar often becomes impacted against the lower second molar; it is common for one or more third molar to fail to develop. Synonym: dens serotinus, molaris tertius, dens sapientiae, wisdom tooth. (05 Mar 2000) |
| third occipital nerve | <anatomy, nerve> Medial branch of the dorsal primary ramus of the third cervical nerve; this is usually joined with the greater occipital, but may exist as an independent nerve supplying cutaneous branches to the scalp and nucha. Synonym: nervus occipitalis tertius. (05 Mar 2000) |
| third-order kinetics | <pharmacology> A term describing the reaction rate of a chemical reaction in which the rate is proportional to the product of the concentrations (in moles) of three of the reactants, the product of the molar concentration of one reactant and the square of the molar concentration of another reactant, or the cube of the molar concentration of one of the reactants. Such a reaction might have an equation like rate = k[A][B][C] or rate = [A][B]2 or rate = [A]3, where k is the reaction rate constant, [A] is the concentration of reactant A, [B] is the concentration of reactant B, and [C] is the concentration of reactant C. (09 Oct 1997) |
| third ovary | An accessory ovary. (05 Mar 2000) |
| third parallel pelvic plane | The plane that extends from the end of the sacrum to the inferior border of the pubic symphysis; it is bounded posteriorly by the end of the sacrum, laterally by the ischial spines, and anteriorly by the inferior border of the pubic symphysis. Synonym: midplane, plane of least pelvic dimensions, plane of midpelvis, third parallel pelvic plane. (05 Mar 2000) |
| third peroneal muscle | <anatomy, muscle> Origin, in common with musculus extensor digitorum longus; insertion, dorsum of base of fifth metatarsal bone; nerve supply, deep branch of peroneal; action, assists in dorsiflexion and eversion of foot. Synonym: musculus fibularis tertius, musculus peroneus tertius, third peroneal muscle. (05 Mar 2000) |
| third sound | Occurs in early diastole and corresponds with the end of the first phase of rapid ventricular filling; normal in children and younger people but abnormal in others. Synonym: third sound. (05 Mar 2000) |
| third stage of labour | The part of labour from the birth of the baby until the placenta (afterbirth) and foetal membranes are delivered. The third stage of labour is also called the placental stage. (12 Dec 1998) |
| third temporal convolution | A sagittal convolution on the inferolateral border of the temporal lobe of the cerebrum, separated from the middle temporal gyrus by the inferior temporal sulcus. On the inferior surface of the temporal lobe it is separated from the medial occipitotemporal gyrus by the occipitotemporal sulcus. It includes the lateral occipitotemporal gyrus. Synonym: gyrus temporalis inferior, inferior temporal convolution, third temporal convolution. (05 Mar 2000) |
| third tonsil | A collection of more or less closely aggregated lymphoid nodules on the posterior wall and roof of the nasopharynx, the hypertrophy of which constitutes the morbid condition called adenoids. Synonym: tonsilla pharyngealis, tonsilla adenoidea, Luschka's gland, Luschka's tonsil, third tonsil. (05 Mar 2000) |
| third trochanter | An occasional process at the proximal end of the lateral lip of the linea aspera of the femur, about on a level with the lesser trochanter, giving insertion to the greater part of the gluteus maximus muscle. See: gluteal tuberosity. (05 Mar 2000) |
| third ventricle | A narrow, vertically oriented, irregularly quadrilateral cavity in the midplane, extending from the lamina terminalis to the rostral opening of the mesencephalic aqueduct. This ventricle communicates at its rostrodorsal corner with each of the two lateral ventricles through the left and right interventricular foramen of Monro. Its narrow roof is formed by the tela choroidea which is attached on either side to the tenia thalami; its lateral wall by the medial surface of the thalamus and, below the hypothalamic sulcus, by the hypothalamus which also forms its floor. In lateral profile, the third ventricle exhibits a number of recesses: in its floor, from before backward, 1) the preoptic recess in the acute angle between the base of the lamina terminalis and the dorsum of the optic chiasm, 2) the infundibular recess extending ventrally into the infundibulum but (in humans) not into the hypophysial stalk, and 3) the mamillary or inframamillary recess caused by the protrusion of the mamillary bodies into the ventricle. From its dorsocaudal corner, the pineal recess extends caudally into the pineal stalk. Synonym: ventriculus tertius, diacele, ventricle of diencephalon. (05 Mar 2000) |
| third ventriculostomy | An operation to establish an opening from the third ventricle to the prechiasmal and interpeduncular cisterns (Stookey-Scarff operation) or from the third ventricle to the interpeduncular cistern (Dandy operation). (05 Mar 2000) |
| aberrant ventricular conduction | Abnormal intraventricular conduction of a supraventricular beat, especially where surrounding beats are normally conducted. Synonym: ventricular aberration. (05 Mar 2000) |