II. The analogous structural modifications of the proximal segment of the colon.
I. CÆCUM PROPER.
The pouch of the Ungulates and Rodents, taking these forms as the typical representatives of the entire group, is usually of very large size compared with the rest of the alimentary canal. Two types are found:
1. Large capacious smooth cæcal pouch of uniform caliber ([Fig. 465], IV, 2). This form is met with in the Muridæ among Rodents and is illustrated in [Fig. 393] showing the cæcum of Mus decumanus, var. albinus, the white rat. [Fig. 392] represents the entire alimentary canal of the meadow mouse, Arvicola pennsylvanicus, and indicates the proportion which the cæcal apparatus bears to the remainder of the intestinal tract. The typical cæcum of the Ungulates is shown in [Fig. 371], taken from Capra ægagrus, the bezoar goat, and in [Fig. 372], taken from a preparation of Boselaphus tragocamelus, the Nilghai.
2. The cæcal pouch is large, markedly crescentic in shape, sacculated, or provided in the interior with a more or less complete spiral valve, and reduced in caliber in the terminal segment, forming at times a pointed appendix ([Fig. 465], IV, 3). This form is encountered typically among certain Rodents, as in Castor fiber, the beaver ([Figs. 381] and [382]), and Erethizon dorsatus, the Canadian porcupine ([Figs. 383] and [384]), but is not confined to this order. Thus cæca of very similar structure are found among the Marsupials, as in Phascolarctos and Cuscus ([Fig. 352]). In some of these forms the terminal reduction of the cæcum is very marked, resulting in a long narrow segment of the pouch tapering to a sharp point. It is significant to note in this connection that in one member of the marsupial order, the wombat (Phascolomys), this tendency to terminal reduction of the pouch has led to the development of a cæcum and appendix identical in structure and arrangement with the corresponding parts of man and the anthropoid apes ([Fig. 354]). This is merely another illustration of the fact, evidenced throughout the entire vertebrate series, that a primal type-form of cæcal apparatus, in responding to the conditions which influence the development of structural modifications, will produce identical specific types in animals otherwise widely separated in the zoölogical series.
Thus again the form of cæcum under discussion, found in many Rodents and certain Marsupials, is encountered in the only Insectivore possessing a cæcum (Galeopithecus) ([Fig. 419]), and in several Lemuroidea among Primates ([Figs. 420]-[425]).
II. Structural Modifications of the Proximal Segment of the Colon Analogous in Their Functional Significance to the Cæcal Apparatus.
In these forms, in addition to the cæcal apparatus proper, certain accessory structural modifications of the adjacent large intestine are developed which possess the physiological significance of the cæcal apparatus in general, since they serve to increase the extent of the intestinal mucous surface and to prolong the period during which the contents of the canal are retained for elaboration and absorption. These modifications, which appear most fully developed in certain Rodents and Ungulates, are of two kinds.
1. The development of the colic mucous membrane in the form of a projecting fold or valve usually surrounding the lumen spirally ([Fig. 465], IV, 1). The significance and phylogeny of this spiral fold has been considered above (cf. [p. 193]). Functionally this reduplication must be regarded as in general equivalent to the cæcal apparatus proper, in producing an increased surface for secretion and absorption and in retarding the movement of intestinal contents. The cæcal pouch evidently acts as a reservoir in which partly digested substances, mixed with the secretions of the small intestine, are retained while the slow processes of digestion and absorption, already inaugurated in the antecedent segment of the canal, are completed. It is reasonable to suppose that the system of projecting mucous folds and reduplications encountered in the colon beyond the cæcum have a similar physiological import. Moreover, in certain forms the cæcum itself is provided with a similar spiral mucous fold, as in the instances already mentioned of Lepus among mammalia ([Fig. 381]) and of the Ostrich among birds ([Fig. 341]). We have seen above (cf. [p. 193]) that the spiral intestinal valve is encountered very early in the vertebrate series, in forms in which the alimentary canal is but slightly, or not at all differentiated, short and straight in its course. In these forms the evident purpose of the spiral fold is to retard the movement of the intestinal contents and to increase the area of the secretory and absorbing surface. As a structural modification possessing this character we saw the fold in the Cyclostomata, Selachians and Dipnœans ([Figs. 310], [466], [467] and [468]) and in certain Ophidians (Python and Anaconda, [Figs. 331] and [469]). Among Mammals it is found in certain Rodentia in two forms: