3. The caudal intestinal arterial branch derived from the aorta is the inferior mesenteric artery supplying parts of the transverse colon, the descending colon, sigmoid flexure and rectum [(Figs. 130], [131], [132], and [133]).
On the other hand in the cases of non-rotation of the intestine as above described in [Figs. 118]-[122], the embryonic type of the intestinal arterial supply persists, as indicated schematically in Fig. 134. Not only the pancreatico-duodenalis inferior, but all the remaining branches to the small intestine are derived from the right side of the superior mesenteric artery. The terminal branches of the main artery supply the ileo-colic junction, while the arterial supply of the large intestine, A. colica dextra and media, are given off from the left side of the parent vessel.
II. Demonstration of Intestinal Rotation in the Cat.—The changes in the relative position of the different intestinal segments and the final disposition of the mesenteries and blood vessels can best be understood by the direct examination of the abdominal contents in an animal whose permanent adult arrangement corresponds to one of the early embryonal human stages, and in which the necessary manipulations can readily be carried out and their results noted.
It is doubtful if the above detailed developmental stages in man can ever be clearly comprehended unless the student will for himself examine the conditions and perform the manipulations in one of the lower mammals.
The necessity of keeping the three dimensions of space in mind and the fact that certain structures during and after rotation cover and obscure each other, make diagrams and drawings unsatisfactory unless the actual examination of the object itself is combined with their study. Fortunately, among the common domestic animals of convenient size easily obtained the cat answers every purpose of this study admirably. The student is earnestly urged to pursue his study of the development and adult arrangement of the human abdominal viscera and peritoneum in the light which the anatomy of this animal can shed on the complicated and obscure conditions encountered in the human subject. The plan of having the opened abdominal cavity of the cat directly side by side with the human subject, while the arrangement of the abdominal viscera and peritoneum is considered, cannot be recommended too highly.
Directions.—After killing the animal with chloroform the abdominal cavity is to be freely opened by a cruciform incision and the skin flaps turned well back and secured in this position. It is well to select a male animal or an unimpregnated female, as the size of the pregnant uterus in the later stages renders the examination of the abdominal viscera and peritoneum more difficult.
Fig. 135.—Abdominal viscera of cat; great omentum raised; intestines turned down and to left. (From a fresh dissection.)
For purposes of careful study and comparison of the vascular relations of the abdomen, it is highly desirable to inject the animal with differently colored gelatine, starch or plaster of Paris mass. The arterial injection can be made through the carotid artery, the systemic venous injection through the femoral vein, and the portal circulation can be filled after opening the abdomen, by injection through the superior mesenteric or splenic veins. Animals prepared in this manner are especially useful for the study of the upper portion of the abdominal cavity and of the peritoneal relations of liver, stomach, spleen, pancreas and duodenum. They may be kept for permanent reference in a 5 per cent. solution of formaline or 50 per cent. alcohol.
After opening the abdominal cavity turn the great omentum up over the ventral surface of the thorax and secure it in this position, thus exposing the underlying intestines completely (Fig. 135). Trace in the first place the entire course of the intestinal tube from the pyloric extremity of the stomach down. It will be noticed that the first portion of the small intestine (duodenum) is freely movable, completely invested by peritoneum and attached to the dorsal midline by a mesoduodenum between the layers of which a portion of the pancreas is seen.