| sinus standstill | Cessation of sinus node activity, marked by absence of normal P waves in the electrocardiogram. Ventricular standstill, cessation of ventricular contractions, marked by absence of ventricular complexes in the electrocardiogram. (05 Mar 2000) |
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| sinus tachycardia | A fast heartbeat (tachycardia) occurring because of rapid firing by the sa node, the natural pacemaker of the heart. Electrical signals initiated in the sa node are transmitted to the atria and the ventricles to stimulate heart muscle contractions heartbeats. Sinus tachycardia is usually a rapid contraction of a normal heart in response to a condition, drug, or disease as, for examples, pain, fever, excessive thyroid hormone, exertion, excitement, low blood oxygen level (hypoxia), or stimulant drugs such as caffeine, cocaine, and amphetamines. However, in some cases, it can be a sign of heart failure, heart valve disease, or other illness. (12 Dec 1998) |
| sinus tarsi | tarsal sinus |
| sinus thrombosis | Formation of a clot in a cerebral venous sinus. (12 Dec 1998) |
| sinus tonsillaris | The depression between the palatoglossal and palatopharyngeal arches occupied by the palatine tonsil. Synonym: fossa tonsillaris, amygdaloid fossa, sinus tonsillaris. (05 Mar 2000) |
| sinus transversus | A paired dural venous sinus that drains the confluence of sinuses, running along the occipital attachment of the tentorium cerebelli and terminating in the sigmoid sinus. Synonym: sinus transversus, lateral sinus. Transverse pericardial sinus, a passage in the pericardial sac between the origins of the great vessels, i.e., posterior to the intrapericardial portions of the pulmonary trunk and ascending aorta and anterior to the superior vena cava and superior to the atria; it is formed as a result of the flexure of the heart tube, partially approximating the great venous and arterial vessels. Synonym: sinus transversus pericardii, Theile's canal, transverse sinus of pericardium. (05 Mar 2000) |
| sinus transversus pericardii | transverse pericardial sinus |
| sinus trunci pulmonalis | The space at the origin of the pulmonary trunk between the dilated wall of the vessel and each cusp of the pulmonic valve. Synonym: sinus trunci pulmonalis. (05 Mar 2000) |
| sinus tubercle | A median protuberance projecting into the embryonic urogenital sinus from its dorsal wall; it is formed from the fused caudal ends of the paramesonephric ducts and is the first evidence of the embryonic uterus and vagina. Synonym: sinus tubercle. (05 Mar 2000) |
| sinus tympani | A depression in the tympanic cavity posterior to the tympanic promontory. Synonym: sinus tympani. Sinus unguis, the deep cleft housing the root of the nail. Synonym: sinus of nail. (05 Mar 2000) |
| sinus urogenitalis | A condition in which the urorectal fold has failed to divide the cloaca of the embryo into rectal and urogenital portions. Synonym: sinus urogenitalis, urogenital sinus. (05 Mar 2000) |
| sinus venarum cavarum | The portion of the cavity of the right atrium of the heart that receives the blood from the venae cavae; it is separated from the rest of the atrium by the crista terminalis. Synonym: sinus venarum cavarum. (05 Mar 2000) |
| sinus venosus | A cavity at the caudal end of the embryonic cardiac tube in which the veins from the intra-and extraembryonic circulatory arcs unite; in the course of development it forms the portion of the right atrium known in adult anatomy as the sinus of the vena cava. Synonym: saccus reuniens. Sinus venosus sclerae, the vascular structure encircling the anterior chamber of the eye and through which the aqueous is returned to the blood circulation. Synonym: circular sinus, Fontana's canal, Lauth's canal, Schlemm's canal, sinus circularis, venous sinus of sclera. (05 Mar 2000) |
| sinus vertebrales longitudinales | longitudinal vertebral venous sinus |
| sphenoidal sinus | One of a pair of paranasal sinuses in the body of the sphenoid bone communicating with the upper posterior nasal cavity or spenoethmoidal recess. Synonym: sinus sphenoidalis. (05 Mar 2000) |