| internal anal sphincter | <anatomy> A smooth muscle ring, formed by an increase of the circular fibres of the rectum, situated at the upper end of the anal canal, internal to the outer voluntary external anal sphincter. This sphincter is maximally-contracted when the rectal ampulla is at rest -empty or relaxed to accommodate a distending faecal mass. It is inhibited with filling of the ampulla, increased distension and peristalsis. Synonym: musculus sphincter ani internus, internal sphincter muscle of anus. (05 Mar 2000) |
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| external anal sphincter | A fusiform ring of striated muscular fibres surrounding the anus, attached posteriorly to the coccyx and anteriorly to the central tendon of the perineum; it is subdivided, often indistinctly, into a subcutaneous part, a superficial part and a deep part for descriptive purposes. Synonym: musculus sphincter ani externus, external sphincter muscle of anus. (05 Mar 2000) |
| abdominal sac | The part of the embryonic celom that becomes the abdominal cavity. (05 Mar 2000) |
| air sac | <zoology> Thin-walled sacs or spaces which function as a part of the respiratory system in birds, fishes, insects, and mammals. The sacs are filled with air and connected with the air passages of the lungs; an air cell. Source: Websters Dictionary (04 Jul 1999) |
| allantoic sac | The dilated distal portion of the allantois; it forms part of the placenta in many mammals. (05 Mar 2000) |
| alveolar sac | Terminal dilation of the alveolar ducts that give rise to alveoli in the lung; a small air chamber in the pulmonary tissue from which the pulmonary alveoli project like bays and into which an alveolar duct opens; in birds, air-containing extensions of bronchi that connect with bone cavities. Synonym: sacculus alveolaris, air sac. (05 Mar 2000) |
| amniotic sac | <anatomy, obstetrics> Sac, enclosing the embryo of amniote vertebrates, that provides a fluid environment to prevent dehydration during development of land based animals. See: amnion. (18 Nov 1997) |
| aneurysmal sac | The dilated wall of an artery in a saccular aneurysm. (05 Mar 2000) |
| aortic sac | In mammalian embryos, the endothelially lined dilation just distal to the truncus arteriosus; it is the primordial vascular channel from which the aortic arches arise and is homologous to the ventral aorta of gill-bearing vertebrates. (05 Mar 2000) |
| rami laterales ramorum dorsalium nervorum cervicalium/thoracalium/lumbalium/sac | Lateral branch of dorsal primary rami of spinal nerves. See: lateral branches. (05 Mar 2000) |
| gestational sac | <radiology> Arises from blastocyst which implants into secretory endometrium surrounded by echogenic trophoblast sac size: 6th week = 17mm;, 8th week = 31 mm, fills chorionic cavity by 12th week visualization of gestational sac, relative to beta-hCG levels: transabdominal scan: greater than 1800 IU/l, transvaginal scan: less than 500 IU/l = 20%, 500-1000 IU/l = 80%, greater than 1000 IU/l = 100%, relative to menstrual age, transabdominal scan: earliest by 5.5 weeks (10-12mm), transvaginal scan: routinely seen by 5-5.5 weeks see: double decidual sac (12 Dec 1998) |
| gestation sac | The gestation sac is the fluid-filled sac in which the foetus develops, visible with an ultrasound exam. (09 Oct 1997) |
| vestibular blind sac | <anatomy> The lower extremity of the cochlear duct, occupying the cochlear recess in the vestibule. Synonym: caecum vestibulare, vestibular blind sac. (05 Mar 2000) |
| vitelline sac | <embryology, ornithology> One of the set of extra embryonic membranes, growing out from the gut over the yolk surface, in birds formed from the splanchnopleure, an outer layer of splanchnic mesoderm and an inner layer of endoderm. (18 Nov 1997) |
| pericardial sac | <anatomy> A tough fibrous membrane that envelops and protects the heart. (27 Sep 1997) |