To the left of the ascending duodenum a portion of the caudal surface of the pancreas will be seen, covered by the continuation of the caudal leaf of the transverse mesocolon into the parietal peritoneum. The consideration of this relation of peritoneum and pancreas will profitably be deferred until we have studied the developmental changes in the region of the dorsal mesogastrium and great omentum.
In the angle between termination of the transverse colon and proximal part of descending colon (splenic flexure) the caudal part of the ventral surface of the left kidney will be felt. The disposition of the peritoneum and its significance is the same as on the right side. Inasmuch as we have already seen that the secondary parietal peritoneum covering the dorsal abdominal wall on each side of the small intestine’s mesenteric attachment is derived from the primitive ascending and descending mesocolon, it will be readily understood why the blood vessels supplying the ascending and descending colon (arteria ileo-colica, a. colica dextra, a. colica sinistra) are placed behind the parietal peritoneum, while the colica media, supplying the transverse colon, runs between the layers of the transverse mesocolon. Originally the same condition obtained for the two vertical colic segments, but with the anchoring of these portions of the large intestine and the adhesion of their mesocola to the parietal peritoneum the blood vessels which formerly ran between the two layers of the membrane, as long as it remained free, now appear as retroperitoneal vessels placed beneath the parietal peritoneum derived secondarily from the mesocola.
This fact must be borne in mind in studying the arrangement of certain folds and fossæ of the parietal peritoneum which are now to be considered.
Duodenal Fossæ. Fossa of Treitz and Retro-peritoneal Hernia.—The peritoneal cavity of the cat can be used to great advantage in order to obtain a clear idea of the formation of these folds and fossæ, whose relation to the so-called “retro-peritoneal hernia” has led to an exaggerated elaboration of minute detail and a somewhat puzzling terminology in human descriptive anatomy.
Fig. 160.—Abdominal cavity of cat, with intestines everted and elevated to show duodenal fold. (From a fresh dissection.)
Fig. 161.—Abdominal viscera of Nasua rufa, brown coaiti. (From a fresh dissection.)
