The same condition is well shown in the intestinal canal of the snapping turtle ([Fig. 113]). The duodenum and colon approach each other very closely at the isthmus and between these points the convolutions of the intestine extend in a wide circle. We will find this approximation of duodenum and colon a feature which persists throughout all the later developmental stages of the higher vertebrates and has an important bearing on the final arrangement of the intestinal canal in the human adult.
Further Changes in the Development of the Human Alimentary Canal. Rotation of the Intestine. Formation of the Segments of the Colon. Final Permanent Relations of the Segments of the Intestinal Tube.—The next important stage leading up to the final adult disposition of the intestine in man and the higher mammals is the rotation of the portions developed from the two limbs of the primitive loop around an oblique axis drawn from the duodeno-colic isthmus to the apex of the loop. The portion of the large intestine, developed from the ascending limb of the loop, moves in the third month to the middle line, coming into contact with the ventral abdominal wall. From here the large intestine passes, ventrad of the jejuno-ileal coils, toward the cephalic end of the abdominal cavity and lies transversely along the greater curvature of the stomach. The growing coils of the small intestine crowd the colon more and more cephalad. In the fourth month the cæcum turns to the right, coming into contact with the caudal surface of the liver, ventrad of the duodenum, and subsequently reaches the ventral surface of the right kidney. As the result of this rotation the ileo-colic junction, cæcum and succeeding portion of the colon are carried from the original position in the distal and left part of the abdomen cephalad and to the right across the proximal (duodenal) portion of the small intestine, while the coils of the jejuno-ileum, developed from the descending limb and apex of the loop, are turned in the opposite direction, caudad and to the left underneath the preceding ([Figs. 114] and [115]). This change in the relative position of the parts of the intestinal tract and the resulting altered bearing of the colon to the duodenum will be best appreciated by considering in the first place the effect of the change on the arrangement of the primitive mesentery and the intestinal vessels, and secondly by repeating actually the rotation in the intestinal tract of a mammal (cat) in which the adult arrangement of the intestine and peritoneum permits us to perform the manipulations and note the result.
I. Effect of Rotation on the Disposition of the Primitive Mesentery and on the Relative Position of Duodenum and Colon, and Consequent Arrangement of the Intestinal Blood Vessels.—It will be appreciated that in [Fig. 111], representing a profile view of the original arrangement, or in [Figs. 107] and [108], showing the intestinal canal of Tamandua, the left layer of the primitive mesentery is turned toward the observer. The membrane is seen to pass from the ventral aspect of the vertebral column and aorta, through the narrow neck of the duodeno-colic isthmus, to expand in the manner already indicated toward its intestinal attachment. In the rotation of the intestine the twist takes place at the duodeno-colic neck, carrying, as already stated, the large intestine cephalad and to the right, while the jejuno-ileum is turned in the opposite direction caudad and to the left. During this rotation the duodeno-jejunal angle (Figs. 114, B and 115, A) passes to the left underneath the proximal segment of the colon, which now lies ventrad and to the right of the duodenal portion of the small intestine. The mesenteric peritoneum, occupying the bight of the umbilical loop, will, after the rotation, in the left profile view shown in [Fig. 104], A and B, turn its original right leaf toward the beholder, i. e., toward the left, while the original left leaf is turned toward the right.
Observation of the difference in the position of the ileo-colic junction will still further accentuate the change in the relative position of the parts which has been effected by the rotation. In the primitive condition shown in [Fig. 104], A, the ileum enters the large intestine from right to left, and the concavity of the cæcal bud turns its crescentic margin ventrad and to the right.
After rotation is accomplished ([Fig. 104], B and C, and Fig. 115) the ileo-colic entrance takes place in the opposite direction, from left to right and the cæcum turns its concave margin caudad and to the left.
Fig. 116.—Abdominal viscera of Tamandua bivittata, with the intestine rotated to correspond to the development in the human subject. (From a fresh dissection.)