This condition is occasionally encountered. Dr. Martin, in a recent examination of the vascular supply of caecum and appendix in one hundred subjects, found it to obtain in six instances. Apparently the dorsal wall of the caecum and of the proximal segment of the ascending colon remains free in these cases and does not become adherent to the parietal peritoneum. The shape of the pouch, moreover, indicates a free and unimpeded embryonal caecal descent. The normal relative size of the two vascular folds is reversed. A good example of this variation, in the caecum of an infant, is seen in Fig. 516. The same arrangement in an adult specimen is seen in Fig. 574. [Illustration: FIG. 574.--Human adult. Caecum and ileo-colic junction; well-developed ventral vascular fold, carrying appendicular artery. (Columbia University Museum, No. 1613.)] In the Slow Lemur (_Nycticebus tardigradus_) (Fig. 420) the ventral artery is normally the larger of the two, extending in the ventral fold to the tip of the reduced appendix of the caecal pouch. (_B_) _Fusion of ventral vascular fold with the intermediate fold, resulting in the production of a well-defined superior or ventral ileo-caecal fossa._ Normally the reduced ventral artery crosses the ileo-colic junction in a slightly developed ventral vascular fold, closely adherent to the intestine, with a very narrow free margin. The superior or ventral ileo-caecal fossa in these cases is very shallow and confined (Fig. 574) to the ventral surface of the ileo-colic junction. Occasionally the fold is better developed and fuses with the intermediate non-vascular fold, producing a fossa of greater extent, which is bounded dorsad by the ileum, ventrad and cephalad by the ventral fold, caudad by the fusion of this fold with the intermediate reduplication, and to the right by the left wall of the caecum. Figs. 576, 577, 578 and 579 show this aberrant disposition of the structures in a series of adult human caeca. [Illustration: FIG. 575.--Human foetus at term. Ileo-colic junction and caecum; ventral view. (Columbia University Museum, No. 1715.)] [Illustration: FIG. 576.--Human adult. Ileo-colic junction and caecum; ventral appendicular artery and ileo-caecal fossa. (Columbia University Museum, No. 1614.)] [Illustration: FIG. 577.--Human adult. Ileo-colic junction and caecum; ventral appendicular artery and ileo-caecal fossa. (Columbia University Museum, No. 1657.)] [Illustration: FIG. 578.--Human adult. Ileo-colic junction and caecum; ventral appendicular artery and ileo-caecal fossa. (Columbia University Museum, No. 1856.)] [Illustration: FIG. 579.--Human adult. Ileo-colic junction and caecum; ventral appendicular artery and ileo-caecal fossa. (Columbia University Museum, Study Collection.)]