The resulting characteristic adult position of the appendix in these cases is as follows:

The termination of the cæcum proper, and the root of the appendix are under cover of the terminal ileum and frequently adherent to the parietal peritoneum of the iliac fossa (Fig. 555). The distal portion of the appendix remains free, either hanging down and in over the brim of the pelvis ([Fig. 542]), or turned upwards and to the left and coiled in several turns ([Figs. 504], 555 and 556).

Fig. 555.—Human fœtus at term. Ileo-colic junction and cæcum; dorsal view. The area of peritoneal adhesion is seen to involve the dorsal aspect of the cæcum as far as the root of the appendix. (Columbia University Museum, No. 1549.)   Fig. 556.—Human infant. Ileo-colic junction and cæcum, with extensive adhesion to parietal peritoneum. (Columbia University Museum, No. 301.)

Finally the erect vertical retro-cæcal position of the appendix presents several important variations in the disposition of the peritoneal investment. In [Fig. 503], taken from an embryo of 7.6 cm. vertex-coccygeal length, the early complete recession of the retro-cæcal ileal convolutions has probably permitted an early apposition and adhesion of the beginning of the colon to the dorsal prerenal parietal peritoneum. The subsequent descent into the iliac fossa produces a bend in the ventral wall of the colic tube, with a marked convexity directed downwards and forwards, the apex of the bend situated at or near the level of the ileo-colic junction, while the dorsal colic wall is held by the adhesion to the parietal peritoneum, thus giving a backward inclination to the entire cæcum and appendix. During the subsequent descent of the cæcum proper this bend in the colon is gradually diminished and the tube becomes straightened but the apex of the cæcum remains turned back and the appendix is placed in a more or less vertical erect position behind cæcum and ascending colon.

Fig. 557.—Human adult. Ileo-colic junction and cæcum; ventral view. (Columbia University Museum, No. 1612.)
Fig. 558.—Same preparation as Fig. 557; dorsal view. The appendix, erected vertically between cæcum and colon, is completely imbedded in connective tissue.

As regards the disposition of the peritoneal membrane in this type of appendix the following conditions are to be noted:

(a) (Schema, [Fig. 552].)—The apex of the cæcum and the entire appendix are extraperitoneal, imbedded in the loose connective tissue which occupies the area of serous obliteration. The line of peritoneal reflection from the dorsal wall of the secondary caput coli to the parietal peritoneum of the right iliac fossa is placed transversely below the true apex of the fœtal cæcum and the root of the appendix. The latter tube, imbedded in connective tissue, passes vertically upwards behind the ascending colon, its tip frequently reaching the ventral surface of the right kidney. A well-marked example of this arrangement in the adult is shown in Figs. 557 and 558 (ventral and dorsal view, with peritoneal reflection and vertical retro-colic appendix).

(b) (Schema, [Fig. 553].)—In other cases, with the same position of the appendix, the entire cæcum and greater part of the ascending colon remains free. The vertically erected appendix is closely attached to the dorsal surface of the ascending colon, included within the serous investment of the large intestine. The adhesion of the latter is confined to a limited area near the hepatic flexure. Consequently cæcum and greater part of ascending colon can be turned up, away from the parietal peritoneum of the iliac fossa, and the dorsal surface of the appendix shows the free serous covering of the adjacent large intestine.

We may assume that this type of the peritoneal relations of the appendix is produced in one of two ways: