The study of the variations of the adult caecum and appendix involves the consideration of the following points: (_a_) Shape of caecum and origin of appendix. (_Type of adult caecum._) (_b_) Position, direction and peritoneal relations of the appendix. (_c_) Arrangement of the vascular and serous ileo-caecal folds. The peculiarities encountered in any individual case usually depend upon the combination of all three of these factors, which together influence and determine the arrangement of the structures in the adult. Hence the examination of each case should be made with reference to these three points, which we will now consider in detail. A. SHAPE OF CAECUM AND ORIGIN OF APPENDIX. TYPES AND VARIATIONS OF ADULT CAECUM AND APPENDIX. The various forms of the adult caecum are all derived by modifications from the foetal type of the pouch. In the embryo the caecum is funnel-shaped, narrowing gradually and symmetrically in caliber to the root of the appendix, at which point the three colic taenia or longitudinal muscular bands of the large intestine meet. The appendix arises from the apex of the funnel, the lateral walls of which are equally and symmetrically developed. The entire pouch is of a crescentic shape, the concavity of the curve turned to the left and directed toward the caudal margin of the terminal ileum. Two subdivisions of the foetal type are found: I. The crescentic curve of the caecum is only slightly marked; the appendix arises from the most pendent part of the pouch and hangs downward (schema, Fig. 509, _I, a_). This form, which is encountered only occasionally in the foetus and infant, is illustrated by the preparation shown in Fig. 510, taken from a foetus at term. [Illustration: FIG. 509.--Schematic table of types of human caeca.] II. In the majority of cases the inherent crescentic shape of the caecal pouch is pronounced and carries the termination of the funnel with the root of the appendix cephalad and to the left toward the caudal margin of the ileo-colic junction (schema, Fig. 509, _II, a_). At birth this typical arrangement of the caecum frequently places the pouch in a nearly transverse position, with the apex and the root of the appendix turned to the left, in contact with, or under cover of the terminal piece of the ileum at its junction with the large intestine. Figs. 511 and 512 represent the parts in the ventral view in the foetus at term. [Illustration: FIG. 510.--Human foetus at term. Caecum and ileo-colic junction; ventral view. (Columbia University, Study Collection.) 1. Appendix. 2. Reduced intermediate non-vascular fold. 3. Ventral vascular fold. ] [Illustration: FIG. 511.--Human foetus at term. Caecum and ileo-colic junction; ventral view. (Columbia University, Study Collection.) 1. Appendix, coiled spirally behind terminal ileum. 2. Non-vascular intermediate fold. ]