THE PERITONEUM. 1749 



in the adult ; but generally, except just below the colon, the two layers fuse into one. 

 In the adult, when the returning fold reaches the transverse colon, the two layers 

 composing it seem to diverge to enclose the intestine, and, reuniting above it, to be 

 continued upward as the transverse mesocolon to a line running across the back of 

 the abdomen, to be described later. This is an extraordinary and apparently con- 

 tradictory arrangement by which a part of the mesogastrium, or mesentery of the 

 stomach, has become also the mesentery of a part of the colon. The explanation 

 is furnished by embryology, since the original arrangement is very different. In 

 the foetus (Fig. 1439) the returning fold of the greater omentum passes up in front 

 of the colon to the posterior wall along the lower border of the pancreas. The pos- 

 terior layer of the greater omentum is in fact the left layer of the original mesogas- 

 trium, which we should be able to follow to the aorta, had it not, with the pancreas, 

 become adherent to the posterior wall. It has no connection whatever with the 

 transverse mesocolon ; it simply lies upon it. At about birth, however, the two 

 apposed layers begin to fuse. The acquired line of attachment to the transverse 

 colon is low on the right and high on the left. Sometimes near the spleen it joins, 

 not the colon, but the mesocolon above it. 



The Structure of the Greater Omentzim. There is hardly any framework apart 

 from the vessels that course through it, save a most delicate layer of fibre-elastic 

 tissue which supports the mesothelium. In the adult more or less fat is found about 

 the vessels, and in some cases the omentum is loaded with it. The two layers of 

 serous membrane are sometimes beautifully distinct ; in other cases no trace of a 

 double origin can be recognized. Sometimes parts of the omentum atrophy and 

 disappear, leaving windows, or fenestrtz, between the meshes of the vessels. The 

 arteries are long and very slender. They arise from the gastro-epiploic arteries at 

 the greater curvature of the stomach and run straight downward to the folded 

 border of the omentum, and then up again in the posterior fold, to anastomose 

 with the arteries of the colon. In their course they send off small side branches 

 which meet those from the next branch. The arrangement of the veins is essen- 

 tially the same. 



The Lesser Cavity of the Peritoneum. The mesogastrium, starting at the aorta, 

 takes a great turn to the left, and its first part, containing the pancreas, fuses with 

 the posterior abdominal wall. This fold is only a part of a great pouch that runs 

 downward also. If examined before it has become adherent to the transverse meso- 

 colon, its continuation from below the pancreas is to be followed down over the 

 colon as the posterior layer of the greater omentum. In the description of the folds 

 of the adult in a sagittal plane it was necessary, on account of this adhesion, to 

 reverse the normal course and to follow it from its insertion into the stomach back 

 to its origin. If a cut be made through the greater omentum between the stomach 

 and the transverse colon, the lesser sac (bursa omentalis) is opened so that its pos- 

 terior wall can be examined (Fig. 1475). This is seen covering the pancreas, the 

 splenic vessels and the posterior abdominal wall, part of the spleen, part of the left 

 kidney, and the left suprarenal capsule. At the right is the foramen of Winslow, 

 which is generally, but inaccurately, considered the communication between the 

 greater and lesser cavities. It cannot be the true entrance into the lesser cavity, 

 because, owing to the median arrangement of the original mesentery, this opening 

 cannot be on the right of the median line. The real communication between the 

 two cavities is somewhat contracted (isthmus bursae omentalis) and indicated by 

 the median vertical fold -plica gastro-pancreatica made by the mesogastrium over 

 the gastric artery of the stomach as it arises from the cceliac axis to the cardia. On 

 the left of this fold is the lesser cavity proper ; on the right of it, extending to the 

 foramen of Winslow, is a small cavity, the vestibule (vestibulum bursae omentalis), 

 bounded behind by the original parietal peritoneum of the right abdominal wall 

 and extending upward behind the lobe of Spigelius (Fig. 1476). The sides of the 

 pocket behind the liver (recessus superior) are the reflections of the peritoneum over 

 the left of the inferior vena cava and the right of the ductus venosus, which meet 

 above, roofing it in. The first part of the duodenum, which forms the lower boundary 

 of the foramen of Winslow, passes backward and upward, so that the loop of intes- 

 tine, which the duodenum originally formed, must be considered as having fallen 



