CUNNINGHAM, •TEXT•BOOK OF ANATOMY," COURTESY OF OXFORD MEDICAL PUBLICATIONS FIG. 3.-CIRCULAR FOLDS IN THE MUCOUS MEMBRANE OF THE SMALL Fig. 3.-CIRCULAR FOLDS IN THE MUCOUS MEMBRANE OF THE SMALL INTESTINE THAT INCREASE THE SECRETORY AND ABSORBENT SURFACE right side. They are known as the plicae recti or valves o f Hous ton. In the anal canal are four or five longitudinal folds called the columns of Morgagni. (For further details, see Quain's Anat omy, 1896; Gray's Anatomy, 19 2 6 ; Cunningham's Anatomy, Edinburgh 1922).
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 Enid 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 f ore gut, a good deal of the colon and the 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 fusi form dilatation in the straight tube. Between the two the oesoph agus 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 connection between the gut and the yolk-sac is the very rare presence (about 2%) of Meckel's diverticulum. 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 AND SEROUS MEMBRANES). The intestine now grows very rapidly and is thrown into a series of coils; the caecum ascends and passes to the right ventral to the duodenum, and presses it against the dorsal wall of the abdomen; then it descends toward its per manent position in the right iliac f ossa.
From the ventral surface on the hinder (caudal) closed end of the intestinal tube the allantois grows to form the placenta and bladder (see URINARY SYSTEM,