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SMALL INTESTINE

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Originally appearing in Volume V01, Page 666 of the 1911 Encyclopedia Britannica.
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INTESTINE X20 From A. See also:Birmingham; See also:Cunningham's See also:Text-See also:Book of See also:Anatomy. and the duodenum. The submucous coat is very strong and consists of loose areolar See also:tissue in which the vessels break up. The mucous coat is thick and vascular (see fig.2); it consists of an See also:epithelial layer most internally which forms the intestinal glands (see EPITHELIAL, ENDOTHELIAL AND GLANDULAR TISSUES). See also:External to this is the See also:basement membrane, outside which is a layer of retiform tissue, and this is separated from the submucous coat by a very thin layer of unstriped muscle called the muscularis mucosae. In the duodenum and jejunum the mucous membrane is thrown into a See also:series of transverse pleats called valvulae conniventes (see fig. 3); these begin about an See also:inch from the pylorus and gradually fade away as the ileum is reached. About 4 in. from the pylorus the See also:common bile and pancreatic ducts See also:form a papilla, above which one of the valvulae conniventes makes a See also:hood and below which a See also:vertical See also:fold, the frenulum, runs downward. The See also:surface of the mucous membrane of the whole of the small intestine has a velvety See also:appearance, due to the presence of closely-set, See also:minute, See also:thread-like elevations called villi (see fig. 2). Throughout the whole length of the intestinal See also:tract are minute masses of lymphoid tissue called solitary glands (see fig.

2); these are especially numerous in the caecum and appendix, while in the ileum they are collected into large See also:

oval patches, known as agminated glands or Peyer's patches, the See also:long axes of which, from See also:half an inch to 4 in. long, See also:lie in the long See also:axis of the bowel. They are always found in that See also:part of the intestine which is furthest from the mesenteric See also:attachment. In the interior of the rectum three shelf-like folds, one above the other, project into the cavity and correspond to the lateral concavities or kinks of the See also:tube. They are not in the same See also:line and the largest is usually on the right See also:side. They are known as the plicae recti or valves of See also:Houston. In the anal See also:canal are four or five See also:longitudinal folds called the columns of See also:Morgagni. (For further details, see See also:Quain's Anatomy, See also:London, 1896; See also:Gray's Anatomy, London, 19o5; Cunningham's Anatomy, See also:Edinburgh, r906.) See also:Embryology.—The greater part of the alimentary canal is formed by the closing-in of the entoderm to make a longitudinal tube, ventral and parallel to the notochord. This tube is See also:blind in front and behind (cephalad and caudad), but the See also:middle part of its ventral See also:wall is for some distance continuous with the wall of the yolk-See also:sac, and this part of the canal, which at first opens into the yolk-sac by a very wide See also:aperture, is called the See also:mid gut. The part in front of it, which lies dorsal to the See also:heart, is the fore gut, while the part behind the aperture of the yolk-sac is the See also:hind gut. The pharynx, See also:oesophagus, See also:stomach and part of the duodenum are See also:developed from the fore gut, a See also:good See also:deal of the See also:colon and the A B From A. Birmingham; Cunningham's Text-Book of Anatomy. A, As seen in abitof jejunum which has been filled with See also:alcohol and hardened.

B, A portion of fresh intestine spread out under See also:

water. rectum from the hind gut, while the mid gut is responsible for the See also:rest. The cephalic part of the fore gut forms the pharynx (q.v.), and about the See also:fourth See also:week the stomach appears as a fusiform See also:dilatation in the straight tube. Between the two the oesophagus gradually forms as the embryo elongates. The opening into the yolk-sac, which at first is very wide, gradually narrows, as the ventral abdominal walls See also:close in, until in the adult the only indication of the connexion between the gut and the yolk-sac is the very rare presence (about 2%) of Meckel's diverticulum already referred to. The stomach soon shows signs of the greater and lesser curvatures, the latter being ventral, but maintains its straight position. About the See also:sixth week the caecum appears as a lateral diverticulum, and, until the third See also:month, is of See also:uniform calibre; after this See also:period the terminal part ceases to grow at the same See also:rate as the proximal, and so the vermiform appendix is formed. The mid gut forms a See also:loop with its convexity toward the diminishing vitelline duct, or remains of the yolk-sac, and until the third month it protrudes into the umbilical See also:cord. The greater curvature of the stomach grows more rapidly than the lesser, and the whole stomach turns over and becomes See also:bent at right angles, so that what was its See also:left surface becomes ventral.

End of Article: SMALL INTESTINE

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