1. Vessels pass from the proximal part of the abdominal aorta to the stomach and pyloric portion of the duodenum. This set of vessels forms the rudiment of the future cœliac axis. With the development of the liver and pancreas by budding from the duodenum, and with the appearance of the spleen in the mesoderm of the dorsal mesentery, branches corresponding to these organs (hepatic and splenic arteries) are added to the gastric and duodenal vessels and the adult arrangement of the cœliac axis is thus obtained (Figs. 130, 131, 132 and 133).

Fig. 130.—Diagrammatic representation of the arteries proceeding to the alimentary canal and appendages prior to rotation of intestine (stage of simple umbilical loop).   Fig. 131.—Diagrammatic representation of the arteries of the alimentary canal in the first stage of intestinal rotation, showing relation of superior mesenteric artery to the transverse portion of the duodenum.
Fig. 132.—Arteries of alimentary canal in the later stages of intestinal rotation. Fig. 133.—Final arrangement of arteries of alimentary canal after completed rotation of the intestines.

These vessels have an important bearing on the formation of the adult peritoneal cavity in the retro-gastric space, and will be considered in detail below with that portion of the subject.

2. The next vessel in order derived from the aorta and supplying the duodenum, pancreas, the small and a part of the large intestine is the above-mentioned superior mesenteric artery, which arises from the aorta a short distance caudad of the cœliac axis (Figs. 130, 131, 132 and 133).

At the time when the intestine still presents the primitive arrangement of the umbilical loop ([Figs. 104] and [130]) this vessel passes between the layers of the dorsal mesentery through the narrow duodeno-colic neck to reach the two limbs and the apex of the intestinal loop. In its course it gives off successively branches to the gut from each side. Those from the right side of the main vessel pass to the duodenum, pancreas, jejunum and ileum. Those from the left side of the main vessel accede in succession to the colic angle of the isthmus, the proximal portion of the colon, the cæcum and the ileo-colic junction. The terminal portion of the superior mesenteric artery supplies the ileum near the ileo-colic entrance. After rotation it will be found that the turn has occurred at the point X ([Fig. 130]), i. e., in that part of the vessel which occupies the duodeno-colic isthmus. Hence it will be found that the first branches derived from the right side of the primitive superior mesenteric artery, supplying the duodenum and pancreas (Art. pancreatico-duodenalis inferior) still arise after rotation from the right side. They are succeeded, beyond the point X, by the original highest left branches passing to colon, cæcum and ileo-colic junction, while all the original right-sided vessels, except the inferior pancreatico-duodenal, appear now as branches from the left side of the main artery, supplying the coils of the jejuno-ileum. Hence in the adult (Fig. 133) the succession of branches derived from the right or concave side of the superior mesenteric artery is as follows:

Fig. 134.—Schematic representation of intestinal arterial supply from superior mesenteric artery in cases of arrested rotation of the intestine.

1. Arteria pancreatico-duodenalis inferior.
2. Arteria colica media.
3. Arteria colica dextra.
4. Arteria ileo-colica.

On the other hand, the first branches from what has now become the left or convex side of the vessel are the original lower right-hand vessels to the small intestine developed from the descending limb of the loop. Hence in the adult the left side of the superior mesenteric vessel gives rise to the vasa intestini tenuis.