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Online Encyclopedia
Originally appearing in Volume V01, Page 666 of the 1911 Encyclopedia Britannica.
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SMALL INTESTINE X20 From A. Birmingham; Cunningham's Text-Book of Anatomy. and the duodenum. The submucous coat is very strong and consists of loose areolar tissue in which the vessels break up. The mucous coat is thick and vascular (see fig.2); it consists of an epithelial layer most internally which forms the intestinal glands (see EPITHELIAL, ENDOTHELIAL AND GLANDULAR TISSUES). External to this is the 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 series of transverse pleats called valvulae conniventes (see fig. 3); these begin about an inch from the pylorus and gradually fade away as the ileum is reached. About 4 in. from the pylorus the common bile and pancreatic ducts form a papilla, above which one of the valvulae conniventes makes a hood and below which a vertical fold, the frenulum, runs downward. The surface of the mucous membrane of the whole of the small intestine has a velvety appearance, due to the presence of closely-set, minute, thread-like elevations called villi (see fig. 2). Throughout the whole length of the intestinal 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 oval patches, known as agminated glands or Peyer's patches, the long axes of which, from half an inch to 4 in. long, lie in the long axis of the bowel. They are always found in that part of the intestine which is furthest from the mesenteric 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 tube. They are not in the same line and the largest is usually on the right side. They are known as the plicae recti or valves of Houston. In the anal canal are four or five longitudinal folds called the columns of Morgagni. (For further details, see Quain's Anatomy, London, 1896; Gray's Anatomy, London, 19o5; Cunningham's Anatomy, Edinburgh, r906.) 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 blind in front and behind (cephalad and caudad), but the middle part of its ventral wall is for some distance continuous with the wall of the yolk-sac, and this part of the canal, which at first opens into the yolk-sac by a very wide aperture, is called the mid gut. The part in front of it, which lies dorsal to the heart, is the fore gut, while the part behind the aperture of the yolk-sac is the hind gut. The pharynx, oesophagus, stomach and part of the duodenum are developed from the fore gut, a good deal of the 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 alcohol and hardened. B, A portion of fresh intestine spread out under water. rectum from the hind gut, while the mid gut is responsible for the rest. The cephalic part of the fore gut forms the pharynx (q.v.), and about the fourth week the stomach appears as a fusiform 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 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 sixth week the caecum appears as a lateral diverticulum, and, until the third month, is of uniform calibre; after this period the terminal part ceases to grow at the same rate as the proximal, and so the vermiform appendix is formed. The mid gut forms a 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 cord. The greater curvature of the stomach grows more rapidly than the lesser, and the whole stomach turns over and becomes bent at right angles, so that what was its left surface becomes ventral. This turning over of the stomach throws the succeeding part of the intestine into a duodenal loop, which at first has a dorsal and ventral mesentery (see COELOM

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