[Illustration: FIG. 114_A_.--Intestinal canal in stage of umbilical loop--before rotation.]
[Illustration: FIG. 114_B_.--First stage in rotation, colon crossing duodenum.]
[Illustration: FIG. 115_A_.--Second stage in rotation--rotation of small intestine.]
[Illustration: FIG. 115_B_.--Schema of intestinal canal after complete rotation and descent of cæcum.]
=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.