Following the duodenum caudad it will be observed that the gut can be traced directly continuous with the remaining coils of the small intestine. The ileo-colic junction and the beginning of the large intestine are marked by a short pointed cæcum. The large intestine is short, as it is in all carnivore mammals, and passes from the cæcum almost directly down into the pelvis.
Take the cæcum and the first portion of the large intestine and turn them caudad and over to the left side as far as the peritoneal connections will permit.
Spread out the coils of the small intestine in the opposite direction, i. e., over to the right side.
The arrangement of the intestinal tract after these manipulations should appear as shown in [Figs. 136] and [137].
Fig. 136.—Abdominal viscera of cat, hardened; omentum removed to display derivation of intestines from umbilical loop and the relation of the superior mesenteric artery and common dorsal mesentery to the small and large intestines. (Columbia University Museum, No 728.)
Fig. 137.—Abdominal cavity of cat. (From a fresh dissection.)
It will be seen that all the essential features described for the corresponding stage in the human embryo ([Fig. 104], A) exist here. The proximal portion of the small intestine (duodenum) retains its freedom and mobility, being attached to the ventral surface of the vertebral column by the portion of the primitive mesentery which now constitutes the mesoduodenum. The gut itself forms a bend with the convexity turned to the right.
Observe in the next place that the point (Fig. 136, X), where small intestine and colon approach each other closely, marks the situation of the fœtal duodeno-colic isthmus. The small intestine at this point corresponds to the future duodeno-jejunal angle as will be seen after rotation has been accomplished.