Figs. 483-485.—Schematic figures illustrating possible line of derivation of aberrant mammalian type of alimentary canal encountered in Hyrax.
We have left for our final consideration the aberrant and unique mammalian type found in Hyrax ([Fig. 378]). In this remarkable little animal the large intestine develops a typical mammalian sacculated cæcum at the ileo-colic junction, and in addition is provided further on with two symmetrical pointed lateral colic cæca of large size. It is quite true that this arrangement is unique among Mammalia, confined entirely to the members of the suborder formed by the single family of Hyrax, and that no strictly analogous disposition of the alimentary canal is encountered in the entire vertebrate series. Yet these aberrant structures are possibly capable of explanation, in regard to the method of their development, by reference to the cæcal apparatus of certain phytophagous saurians, as Iguana and Cyclura. In these forms ([Fig. 326]-[330]) the small intestine enters the colon somewhat asymmetrically, the opening being guarded by a well developed annular valve.
The proximal segment of the large intestine forms an extensive sacculated pouch. If this is opened ([Figs. 328]-[330]) it is seen that the small intestine leads into a compartment which is separated from the remainder of the pouch by a valvular diaphragm with central circular opening. Beyond this primary compartment the colic pouch is incompletely subdivided by a series of gradually diminishing crescentic folds, corresponding to the external constrictions between the sacculations. The entire pouch gradually diminishes in caliber until it passes with a sharp angular bend into the terminal portion of the endgut. This terminal segment is differentiated from the elongated colic pouch by the greater thickness of its muscular walls and by a slight annular projecting fold in the interior. In considering the intestinal tract of Hyrax it is conceivable that the unique condition presented by this animal may be derived from some type conforming in general structure to the reptilian arrangement of the parts just detailed, as indicated in the schematic Figs. 483-485. The proximal typical cæcal pouch of Hyrax would then correspond to the similar colic pouch of Iguana. To explain the supplementary colic cæca it is necessary to suppose that the transition of the colic pouch into the terminal hindgut had become well differentiated, and that on each side of this junction the colic tube had extended backwards, resulting in the production of the supplementary bilateral cæcal pouches of Hyrax.
PART IV.
MORPHOLOGY OF THE HUMAN CÆCUM AND VERMIFORM APPENDIX.
Not only is the anatomy of this portion of the alimentary tract of great interest in relation to the evolution of the human structure, but in addition the pathological and surgical importance of the region warrants a very careful study of the cæcum and appendix. This is more especially the case since a number of variations in the arrangement of the structures are encountered. These departures from what we consider the normal human type have an important bearing on the development and progress of the pathological conditions prone to involve the appendix. We may consider the subject under the following subdivisions:
I. DEVELOPMENT OF THE CÆCUM AND APPENDIX.