Hyrtl has observed the same variation. An example of partial persistence of the omphalo-mesenteric artery in the adult is well seen in the case of Meckel's diverticulum shown in Fig. 37, where the arterial vessel continued upon the diverticulum represents the embryonic omphalo-mesenteric artery. The remaining intestinal arteries are at first more numerous and paired. In man and most mammals they are early reduced in number, passing from the abdominal aorta to the dorsal or attached border of the intestine, between the two peritoneal layers of the primitive dorsal mesentery (Fig. 104). The arterial blood supply of the intestinal canal then presents three general divisions: 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 coeliac 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 coeliac axis is thus obtained (Figs. 130, 131, 132 and 133). [Illustration: FIG. 130.--Diagrammatic representation of the arteries proceeding to the alimentary canal and appendages prior to rotation of intestine (stage of simple umbilical loop).] [Illustration: 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.] [Illustration: FIG. 132.--Arteries of alimentary canal in the later stages of intestinal rotation.] [Illustration: 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 coeliac axis (Figs. 130, 131, 132 and 133).