Fig. 273.—Schematic profile view of ventral mesogastrium with developing liver.
Fig. 274.—Schematic transection of abdomen in region of ventral mesogastrium.
This difference in the extent of the mesogastria is perhaps best understood by reference to their relation to the first portion of the duodenum. We have seen that the duodenum in the early stages is attached dorsally by a portion of the common dorsal mesentery, which, after differentiation of the intestinal tract, immediately follows the dorsal mesogastrium proper, forming the mesoduodenum ([Fig. 172]). The proximal portion of the duodenum (hepatic angle) is still included within the fold of the ventral mesogastrium which membrane terminates immediately beyond this point in the free edge surrounding the umbilical vein (subsequent round ligament) ([Fig. 172]). The remainder of the duodenum is devoid of any ventral attachment, being only connected to the dorsal body wall by the mesoduodenum ([Fig. 197]).
Subsequently, after the fourth month, while the right surface of the mesoduodenum and descending duodenum adhere to the parietal peritoneum, the peritoneal investment of the first portion or hepatic angle remains free. This peritoneal covering of the proximal duodenal segment is situated at the point where the caudal end of the ventral mesogastrium, after surrounding the first portion of the duodenum, becomes continuous with the dorsal mesentery forming the mesoduodenum. Obliteration of the latter membrane by adhesion to the parietal peritoneum leaves the first portion of the duodenum invested on both surfaces by the lesser omentum, derived from the ventral mesogastrium. The ventral surface of the gut is covered by the ventral layer, the dorsal surface by the dorsal layer of the lesser omentum. These two layers become continuous around the right free edge of the lesser omentum (hepato-duodenal ligament) forming the ventral boundary of the foramen of Winslow (cf. infra, [p. 177]).
Returning to the schematic consideration of the ventral mesogastrium above outlined (Figs. 273 and 274) we have to note the first important change in the arrangement depending upon the development of the liver. This organ, growing, as we have seen, from the duodenum, extends between the two layers of the ventral mesogastrium, receiving a serous investment from the same. At an early period the liver, developing thus between the mesogastric layers, reaches the septum transversum and becomes closely connected with it, laying the foundation for the subsequent extensive attachment of the gland to the diaphragm.
Extending caudad the liver grows beyond the caudal free edge of the ventral mesogastrium on each side, carrying the serosa with it. Consequently the ventral margin of the liver becomes indented at this point; the umbilical vein and subsequently its fibrous remnant, the round ligament, are imbedded in a notch and fissure (umbilical notch and fissure) continued from the ventral margin dorsad along the caudal surface of the liver ([Fig. 259]).
This growth of the liver has now effected a division of the primitive ventral mesogastrium into two segments:
1. Ventral portion, between diaphragm and liver, forms the broad falciform or suspensory ligament of the liver.