LARGE AND SMALL INTESTINE, ILEO-COLIC JUNCTION AND CÆCUM.
In considering the anatomy of the human cæcum and vermiform appendix many structural conditions are encountered which can only be correctly appreciated in the light of the physiology of the digestive tract. The alimentary canal as a whole affords one of the most striking examples of the adaptation of structure to function. The constant renewal of the tissues of the body by the absorption of nutritive material, the necessary concomitant egestion of undigestible remnants, the variety in the quantity and character of the food habitually taken, all serve to explain why the alimentary canal responds structurally in individual forms so completely to the physiological demands made upon it. This will become especially evident if we extend our observations to include, in addition to man, a review of the corresponding structures in representative types of the lower vertebrates. Moreover the human cæcum and appendix are in part rudimentary structures, representing a portion of the alimentary tract which, in accordance with altered conditions of food supply and nutrition, has lost its original functional significance to the organism and which consequently exhibits the wide range of structural variation which characterizes the majority of rudimentary and vestigial organs.
The vermiform process of man and the higher primates is thus one of several indications given in the structure of the alimentary canal (the character of the dentition is another example) which suggests that at one phylogenetic period the forms composing the order or their immediate ancestors were largely or entirely herbivorous, and hence possessed a more extensively developed cæcal apparatus than their omnivorous descendants of to-day. In approaching, therefore, the study of the human cæcum and appendix we will at once meet with conditions which call for the simultaneous physiological and morphological consideration of the adjacent small and large intestine.
Again many of the structural peculiarities which characterize the human cæcal apparatus can only be correctly valued by comparison with the corresponding parts in the lower vertebrates. Our inquiry will, therefore, most profitably include the following subdivisions of the subject:
I. General review of the functional and structural characters of the vertebrate large and small intestine.
II. Systematic consideration of the ileo-colic junction and the connected structures in the vertebrate series.
III. Phylogeny of the types of vertebrate ileo-colic junction and cæcum, and their probable lines of evolution.
IV. Detailed morphology of the human cæcum and vermiform appendix.
I. GENERAL REVIEW OF THE MORPHOLOGY AND PHYSIOLOGY OF THE VERTEBRATE INTESTINE.
We have seen that the intestinal tube of all vertebrates is the product of two of the embryonal blastodermic layers, the entoderm and mesoderm. The former furnishes the characteristic and cardinal elements of the digestive tract, viz., the secretory and absorbing epithelium of the mucous membrane and of the accessory digestive glands, the liver and pancreas.