1. The increase in the caliber of the large intestine is chiefly marked along the border opposite to the mesenteric attachment, resulting in a greater degree of convexity in this part of the intestinal wall ([Fig. 465], II, 1). Among Reptilia this condition is found in the ileo-colic junction of some of the pond-turtles, as Pseudemys elegans ([Fig. 323]), while a mammalian example is furnished by the three-toed sloth, Arctopithecus marmoratus ([Fig. 363]).
2. An increase of this lateral extension of the colon leads to the formation of a single lateral cæcal pouch ([Fig. 465], II, 2) such as is seen in another Edentate, Tamandua bivittata ([Fig. 364]), among Mammalia, and in certain Ophidians among Reptiles, as in the Anaconda ([Figs. 331] and [332]).
3. Prolongation of the pouch and reduction in caliber lead to the formation of the slender lateral cæcum found in all the Monotremes ([Figs. 345]-[347], [Fig. 465], I, 3). In its general appearance the cæcum of these singular animals bears a close resemblance to the cæcal pouches of many birds.
4. Direct continuity of small and large intestine, with lateral colic cæcum, extending along the convex free border of the terminal ileum and slightly convoluted at the extremity ([Fig. 465], II, 4), characterizes the entire group of the Cebidæ among the new-world monkeys. The cæcum in these animals is a comparatively long pouch, nearly equalling in caliber the remainder of the intestine, occupying a distinctly lateral position, with the terminal portion rounded and slightly recurved ([Figs. 453] and [454]).
5. The Cynoid group of Carnivora, including the dogs, wolves, jackals and foxes, presents a similar relative position of small and large intestine and cæcum ([Fig. 465], II, 5). The cæcum, compared with that of Cebus, is longer and more highly convoluted ([Fig. 397]). Variations encountered in certain forms indicate reversions to a more primitive type. Thus [Fig. 398] shows the usual form in the dog, while [Fig. 399] exhibits an occasional type in the same animal. The cæcum here is less twisted and indicates the probable derivation of the more commonly encountered type.
III. Rectangular Ileo-colic Junction with Direct Linear Continuity of Cæcum and Colon. ([Fig. 465], III.)
The third general group, to which the large majority of Mammalia belong, is characterized in its typical form by a right-angled entrance of ileum into large intestine and by the direct caudal prolongation of the colon into a cæcal pouch of nearly uniform caliber with globular termination. The axes of the small and large intestine are not in the same line as in the two former groups, but are placed nearly at right angles to each other. With this change in the direction of the main intestinal segments the cæcum ceases to be a lateral appendage to the canal and appears as a caudal prolongation of the colon beyond the ileo-colic junction ([Fig. 465], III). The type-form of this group is encountered among the herbivorous Marsupialia, such as the kangaroos and opossums. [Fig. 350] shows the ileo-colic junction and cæcum in the rock wallaby, Halmaturus derbyanus, and [Fig. 348] the same structures in our common opossum, Didelphis virginiana. The majority of the remaining mammalian forms depend upon modifications of this type, either in the direction of reduction of the cæcal apparatus, or of increased development with concomitant structural changes of similar physiological import in the proximal portion of the colon.
The following subdivisions of the general group may be established.
A. 1. The cæcum is long, markedly curved or uncinate, with the crescentic medial margin turned toward the free border of the terminal ileum. The entire pouch usually diminishes gradually in caliber to its termination ([Fig. 465], III, A, 1). This type is encountered in a large group of new-world monkeys, including the marmosets and howlers.