2. The dorsal portion, between liver and stomach, forms the lesser or gastro-hepatic omentum.
The caudal free edge of the ventral mesogastrium extends between the umbilicus and the caudal surface of the liver, carrying the umbilical vein between its layers. The growth of the liver serves to bury this free edge and the contained vein in a fissure on the caudal surface of the liver. The same obtains in the case of the ductus venosus continued from the umbilical vein (umbilical fissure and fissure of ductus venosus of adult liver). Consequently the original continuity of the broad ligament and lesser omentum, as parts of the primitive ventral mesogastrium, is not readily seen in the adult.
The broad ligament extends across the convex cephalic surface of the liver uniting it to the ventral abdominal wall and diaphragm, while its free falciform edge apparently stops at the umbilical notch in the ventral border of the organ. Actually, however, the obliterated vein is surrounded in the bottom of the fissure, by a peritoneal fold which effects the junction between broad ligament and lesser omentum.
We will see later in what way the permanent adult arrangement of the lesser omentum is brought about. For the present we can state, on the hand of the schematic [Fig. 273], that the free caudal edge of the falciform ligament containing the umbilical vein, and the free edge of the gastro-hepatic omentum form together originally the caudal free edge of the ventral mesogastrium, which membrane becomes separated, by the growth of the liver, into suspensory or broad ligament and lesser or gastro-hepatic omentum.
Fig. 275.—Abdominal viscera of Iguana tuberculata. (Columbia University Museum, No. 1313.)
This primitive disposition of the ventral mesogastrium and the viscera connected with the same, is well shown in some of the lower vertebrates in whom the development never proceeds beyond the early mammalian stages. Fig. 275 shows in profile view from the right side the situs viscerum and peritoneum in Iguana tuberculata.[5] The two dorsal aortic roots are seen to unite to form the main aorta, which descends between the layers of the dorsal mesentery, sending branches to the dorsal margin of œsophagus and stomach. From the opposite border of the stomach the ventral mesogastrium is derived. Its dorsal segment (gastro-hepatic omentum) connects liver and stomach, carrying between its layers the portal vessels, hepatic artery and biliary duct. The ventral segment of the membrane, forming the suspensory or broad ligament, extends between abdominal wall and ventral surface of the liver. Caudad, the lesser omentum and the suspensory ligament are seen to have a common concave falciform edge.
The ventral abdominal vein ascends between the layers of the suspensory ligament and near the liver becomes connected by a large branch with the portal vein. A few smaller branches are seen passing from the abdominal wall beyond this point. In this reptile, therefore, the permanent vascular arrangement corresponds to an early human embryonic stage.
The reptilian ventral abdominal vein is the homologue of the umbilical vein of the placentalia. The large branch passing to the portal vein represents the connection established in the human embryo between the umbilical and portal veins. The small branches, continuing cephalad between the mesogastric layers, represent the temporary proximal remnants which in the human embryo the umbilical veins form in connection with abdominal walls. The permanent adult arrangement of this part of the vascular system in this animal corresponds therefore to one of the stages of development in the human embryo, as previously indicated (cf. [p. 149]; [Figs. 251] and [252]).
PERITONEAL RELATIONS OF LIVER.