[Illustration: FIG. 165.--Abdominal viscera of human fœtus at term, arranged to show duodenal folds and fossa. The jejuno-ileum, ascending and transverse colon have been removed. (Columbia University Museum, No. 1819.)]
In some subjects the “inferior” fold is the only one found, and the only duodenal fossa then encountered looks cephalad. This condition, when associated with the course of the inferior mesenteric vessels in the free edge of the fold, constituted the classical “fossa duodeno-jejunalis” of Treitz, and is described as “Treitz’s fossa.”
Fig. 166 shows the condition in which only a small inferior fold attaches itself to the termination of the transverse duodenum. There is practically an entire absence of duodenal or duodeno-jejunal folds and fossæ. The inferior mesenteric vessels course under cover of the mesocolic secondary parietal peritoneum, but do not produce a fold.
[Illustration: FIG. 166.--Abdominal viscera of human fœtus at term. (Columbia University Museum, No. 1820.)]
[Illustration: FIG. 167.--Abdominal viscera of adult human subject, showing duodenal folds and fossa. (From a fresh dissection.)]
Fig. 167, from an adult human subject, illustrates the further development of the fossa from the fœtal conditions shown in Fig. 165. The well-marked duodenal fossa is bounded by a superior and inferior duodenal fold, uniting laterally in a crescentic margin containing a segment of the inferior mesenteric vein and colica sinistra artery. The lower division of the peritoneal recess thus produced corresponds to the typical (vascular) “fossa of Treitz.” Mesally the projection of the fourth portion of the duodenum bounds the fossa.
In Fig. 168, also taken from an adult human subject, an extensive duodenal recess is bounded in the same way by a superior and inferior duodenal fold. In the interior of the fossa a third duodenal reduplication of the peritoneum (“intermediate duodenal fold”) is seen, as is also the trunk of the inferior mesenteric vein, while the main trunk of the colica sinistra artery courses laterally behind the secondary mesocolic parietal peritoneum near the margin of the descending colon.
[Illustration: FIG. 168.--Abdominal viscera of adult human subject, showing duodenal folds and fossa. (From a fresh dissection.)]