It will be seen in all these instances that neither portal vein nor bile-ducts limit the foramen caudad. These structures can be lifted up and turned toward the median line with the free duodenum and mesoduodenum. But the hepatic artery must pass to the liver from the _retroperitoneal coeliac axis_. In doing this the vessel traverses the cephalic border of the pancreas, and the pyloric extremity of the stomach and duodenum, to reach the lesser omentum which conveys it to the liver. Consequently there must always be a narrow peritoneal neck between the liver cephalad, aorta dorsad, hepatic artery caudad, and pyloric extremity of stomach and duodenum together with the lesser omentum ventrad. It should be remembered that the vessel which extends after the development of the liver into the lesser omentum as the _hepatic_ artery, was originally destined for the supply of these latter structures. In the adult these primary embryonic terminal branches to the intestine appear as secondary branches derived from the hepatic as the main vessel. Their origin, however, serves to keep the beginning of the small intestine in comparatively close connection with the hepatic artery which courses over the dorsal surface of the duodenum to reach the liver. The narrow space thus left between aorta, hepatic artery, duodenum, lesser omentum and liver forms the framework of the foramen of Winslow and appears always as a confined and narrow channel. This relation is shown in the accompanying schematic Figs. 294 and 295 which represent a sagittal section through the foramen. This primitive foramen is thus bounded cephalad by the liver (caudate lobe, connecting Spigelian and right lobes), ventrad by the first portion of the duodenum and the lesser omentum, with hepatic artery behind the intestine and between the omental layers; dorsad by the abdominal background and large retroperitoneal vessels, and caudad by the coeliac axis and beginning of the hepatic artery. [Illustration: FIG. 294.--Schematic sagittal section through foramen of Winslow before fixation of pancreas by adhesion of mesoduodenum.] [Illustration: FIG. 295.--The same section after adhesion of mesoduodenum and pancreas. The pancreas appears secondarily retroperitoneal, after adhesion of apposed surfaces of mesoduodenum and primitive parietal peritoneum over dotted area, producing fixation of dorsal surface of pancreas.]