Fig. 460.—Troglodytes niger, chimpanzee. Ileo-colic junction and cæcum; ventral view. (Drawn from Columbia University Museum preparation No. 675.) 1. Appendix. 2. Intermediate non-vascular ileo-cæcal fold. 3. Colon.   Fig. 461.—Troglodytes niger, chimpanzee. Dorsal view. (Drawn from Columbia University Museum preparation No. 675.) 1. Appendix. 2. Intermediate non-vascular ileo-cæcal fold. 3. Dorsal vascular fold.

4. Chimpanzee, Troglodytes niger.—Figs. 460 and 461 represent the ventral and dorsal view respectively of the cæcum and ileo-colon in a young specimen.

The cæcum is curved to the left and the lowest point of the pouch is formed by the right lateral and ventral wall of the gut, but the extreme crescentic bend which carries the origin of the appendix up and to the left behind the ileo-colic junction is not yet developed in the young animal; on the other hand this character of the cæcum is typically apparent in Figs. 462 and 463, taken from an adult individual of the same species.

Fig. 462.—Troglodytes niger, chimpanzee. Ileo-colic junction and cæcum; ventral view. (Drawn from Columbia University Museum preparation No. 1083.) 1. Ventral vascular ileo-cæcal fold.   Fig. 463.—Troglodytes niger, chimpanzee. Ileo-colic junction and cæcum; dorsal view. (Drawn from Columbia University Museum preparation No. 1083.) 1. Appendix.

Fig. 464.—Troglodytes niger, chimpanzee. Ileo-colic junction and cæcum. (Drawn from Columbia University Museum preparation No. 1525.)

This extreme curve is well seen in the ventral view in Figs. 462 and 464, the latter taken from a large adult specimen. Seen from behind in Fig. 463 the sharp bend or kink in the lumen of the cæcal pouch produced by this curve is striking and resembles the arrangement of certain types of adult human cæca ([p. 247]).

II. PHYLOGENY OF THE TYPES OF ILEO-COLIC JUNCTION
AND CÆCUM IN THE VERTEBRATE SERIES.

The segments of the alimentary canal illustrate very clearly the adaptation of structure to function. Diversity of kind and quantity of food habitually taken and variations in the rapidity of tissue metabolism produce marked morphological modifications in different forms. This is more especially the case with the junction of the mid- and hindgut, the site of development of the cæcal apparatus and of structural alterations of the large intestine possessing a similar physiological significance. No other portion of the visceral tract, with the possible exception of the stomach, illustrates more completely the result of physiological demand on the development of anatomical structure and the morphological possibilities of departure, progressive and retrograde, from a common primitive type in accordance with varying conditions of alimentation.

In coördinating, from the morphological standpoint, the structural differences encountered in this segment of the alimentary canal, two facts become apparent.