1. In the first place the serial study of the ileo-colic junction, as we can briefly define the region in question by borrowing the terminology of anthropotomy, reveals a limited number of principal structural types from which by successive gradations the vast variety of individual forms may be derived.
Fig. 465.—Schematic table of the vertebrate types of ileo-colic junction.
(In the schematic Fig. 465 the fundamental types and their derivatives are indicated. In the following the individual forms illustrating these types are referred to this schema in brackets.)
2. The observer will be impressed by the fact that representatives of all the main types of ileo-colic junction are found within a very limited zoölogical range, as within the confines of a single order. Examples of this are furnished by the Marsupialia and, to a lesser extent, by the Edentata. The members of these zoölogical groups, while united by certain common anatomical characters, such as the reproductive system and dentition, differ widely in habit and in the kind and quantity of the food normally taken. These differences in the method of nutrition have impressed their influence on the structure of the alimentary canal and have led to the evolution of varying and divergent types of ileo-colic junction. The study of this segment of the intestinal tract can therefore elucidate the mutual relationship of the vertebrate groups only to a limited degree and in special cases. On the other hand, it renders very clear the fundamental structural ground-plan common to all vertebrates and accentuates the specialized modifications of this plan which develop in response to the physiological environment. Moreover, such a review serves to reveal the significance of rudimentary and vestigial structures, such as the human vermiform appendix and the serous and vascular folds connected with the same. Throughout the entire vertebrate series the alimentary canal is found to respond with great readiness in its structure to varying grades of functional demand. This fact becomes still more apparent if the inquiry is not limited strictly to the region of the ileo-colic junction but takes into account likewise the structural modifications of similar physiological significance in other segments of the alimentary tract.
A cæcal pouch or diverticulum in some form at the junction of mid- and hindgut is a very common and widely distributed mammalian character. The activity of the tissue-changes in warm-blooded animals, and the consequent necessity for a rapid and complete digestive process, account for the structural modifications of the alimentary tract so commonly encountered among these forms. On the other hand, in the lower cold-blooded vertebrates, notably in fishes and amphibians, the metabolism is slow and the alimentary canal usually simple.
Specifically, the cæcum appears as a pouch or diverticulum in which food-substances, already partially digested and mixed with the secretions of the small intestine, are retained until their elaboration is completed and the nutritive value of the food ingested is secured for the organism. Consequently the most complicated and highly developed cæcal apparatus is found among mammalia in the Herbivora, such as the Ungulates and Rodents, whose food contains a comparatively small amount of nutriment in ratio to its bulk, and hence requires considerable elaboration before absorption. On the other hand the cæcum appears as a reduced or even rudimentary organ, or defaults entirely, in Carnivora whose food is concentrated and easily assimilated, containing only a small amount of non-nutritive material.
The function of the cæcal apparatus may be defined as follows:
Fig. 466.—Squalus acanthius, dog-fish. Alimentary tract, spleen, pancreas. (Drawn from Columbia University Museum preparation No. 1405.)