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 caecal pouch of _Hyrax_ would then correspond to the similar colic pouch of _Iguana_. To explain the supplementary colic caeca 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 caecal pouches of _Hyrax_. [Illustration: FIGS. 483-485.--Schematic figures illustrating possible line of derivation of aberrant mammalian type of alimentary canal encountered in _Hyrax_.] PART IV. MORPHOLOGY OF THE HUMAN CAECUM 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 caecum 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 CAECUM AND APPENDIX. Much light is thrown on the adult anatomy of the parts and on the origin of the variations observed by the study of their embryonic history. In considering the factors which determine the variations in the position, size, and shape of the appendix it must be remembered that the rudimentary character of this structure is responsible for many of the aberrant conditions encountered.