[Illustration: FIG. 128.--Adult human subject with non-rotated cæcum, the ileum entering large intestine from the right and behind, and the appendix placed to the right of the ascending colon. (From a fresh dissection.)]
In Fig. 127 the terminal ileum is sharply bent on itself and adherent to the prerenal parietal peritoneum. It passes from right to left and downwards to enter the right posterior circumference of the large intestine. The cæcum is turned cephalad and the appendix is in contact with the right lobe of the liver. The cæcum passes with a sharp bend into the obliquely directed ascending colon.
In Fig. 128 the ileum enters the colon from the right and below. The apex of the cæcum is turned cephalad and to the right and the appendix extends beneath peritoneal adhesions along the lateral border of the proximal segment of the colon.
In the next place it is desirable to clearly understand the vascular supply of the intestine before and after rotation and the final relation of the superior mesenteric artery to the transverse portion of the duodenum.
The thoracic and abdominal aortæ are at first double, the first aortic arches continuing as so-called “primitive aortæ” ventrad of the vertebral column to the caudal end of the body.
The cephalic portions of the two vessels unite in the chick on the third day and from this point fusion into a single vessel proceeds slowly caudad.
In the rabbit the fusion of the primitive aortæ begins on the ninth day in the region of the lung-buds and progresses from here caudad until by the sixteenth day a single aorta is formed (Fig. 129).
[Illustration: FIG. 129.--Diagrams illustrating the arrangement of the primitive heart and aortic arches. (After Heisler, modified from Allen Thompson.)]
That the entire descending aorta in man results from the fusion of two vessels is shown by the rare cases in which the aorta is divided throughout its entire length by a septum.
The arteries of the allantois are originally the terminations of the primitive aortæ. After fusion of the primitive aortæ to form the abdominal aorta the allantoic arteries, now passing as the umbilical arteries to the placenta, appear as the branches of bifurcation of the abdominal aorta, in the same way as the common iliacs do in the adult.