An anatomical specimen is described in which a considerable part of the small bowel had, apparently, in its return to the abdomen during fetal development invaginated the descending mesocolon and that of the left half of the transverse colon, thus producing a large internal hernia. The ascending branch of the left colic artery and the inferior mesenteric vein were in the neck of the sac, anterior to the loops of bowel entering and emerging from the sac. These relationships were obscured by the fact that there was a broad adhesive band extending from the mesentery of the free loops of small intestine to the descending colon, thus bridging over the vessels. The relationships referred to and as shown in the drawings, which include a cross-sectional view, would indicate that the mechanism of production of the hernia was as suggested and that there were in fact four layers of peritoneum in the anterior wall of the sac.