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 caecum 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 foetal caecum 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). [Illustration: FIG. 557.--Human adult. Ileo-colic junction and caecum; ventral view. (Columbia University Museum, No. 1612.)] [Illustration: FIG. 558.--Same preparation as Fig. 557; dorsal view. The appendix, erected vertically between caecum and colon, is completely imbedded in connective tissue.] (_b_) (Schema, Fig. 553.)--In other cases, with the same position of the appendix, the entire caecum 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 caecum 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: 1. Either the retro-colic appendix has become early attached to the adjacent large intestine, whose dorsal surface in large part remains free, or 2. The arrangement of the peritoneum indicated in schema, Fig. 552, may be subsequently changed into that shown in schema, Fig. 553, by a continued downward displacement of the caecum, producing a secondary serous investment of the dorsal surface of appendix and part of ascending colon. Examples of this type are found both in infantile and adult subjects. In Fig. 538, taken from an infant three years of age, the caecum is lifted up to show the vertical position of the appendix behind the caecum and ascending colon, the dorsal surface of the large intestine retaining its free serous covering. Another illustration of this arrangement in a juvenile subject is shown in Fig. 529. The same condition in the adult subject is illustrated in Figs. 539 and 540.