| sinus septum | A small fold forming the medial end of the valve of the inferior vena cava; it is developed from the dorsal wall of the embryonic sinus venosus. (05 Mar 2000) |
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| sinus sigmoideus | The S-shaped dural venous sinus lying deep to the mastoid process of the temporal bone and immediately posterior to the petrous temporal bone; it is continuous with the transverse sinus and empties into the internal jugular vein as it passes through the jugular foramen. Synonym: sinus sigmoideus. (05 Mar 2000) |
| sinus sphenoidalis | 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) |
| sinus sphenoparietalis | A paired dural venous sinus beginning on the parietal bone, running along the sphenoidal ridges and emptying into the cavernous sinus. Synonym: sinus sphenoparietalis, Breschet's sinus, sinus alae parvae. (05 Mar 2000) |
| 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) |
| 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) |