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 fossae of the parietal peritoneum which are now to be considered. =Duodenal Fossae. 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 fossae, 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. =Directions for Examining the Folds and the Formation of the Duodeno-jejunal Fossa in the Cat.=--Turn the omentum and the coils of the small intestine cephalad out of the abdomen until they rest upon the ventral thoracic wall. Press the large intestine over to the left side, putting the mesocolon on the stretch until the parts are arranged as shown in Fig. 160. The loop of the duodenum with the head portion of the pancreas will be seen caudad of the liver and ventrad of the right kidney. A well-marked peritoneal fold, somewhat sickle-shaped, with the concavity of the free edge directed caudad and to the right, will be seen extending from the convex border of the duodenum, directly opposite the mesenteric or attached margin, to the right leaf of the mesocolon. This fold indicates the beginning adhesion of the duodenum to the mesocolic peritoneum, the first step toward the subsequent complete fixation of the gut as it is found in man. [Illustration: FIG. 160.--Abdominal cavity of cat, with intestines everted and elevated to show duodenal fold. (From a fresh dissection.)] [Illustration: FIG. 161.--Abdominal viscera of _Nasua rufa_, brown coaiti. (From a fresh dissection.)]