(_c_) (Schema, Fig. 554.)--Occasionally, with the appendix erected vertically behind the ascending colon, the apex of the caecum and the proximal portion of the appendix are invested by peritoneum for a short distance and the tip of the appendix likewise obtains a free serous investment, while the intermediate greater portion of the appendix and the corresponding segment of the dorsal surface of the ascending colon are extraperitoneal, adherent to the abdominal parietes. Examples of this peritoneal relation of the appendix in an infant are shown in Figs. 559 and 560, while Fig. 509 represents the same arrangement in an adult specimen. The condition is produced from the arrangement of schema, Fig. 554, by secondary adhesion and obliteration of the serous surfaces over the intermediate portion of the retroverted appendix and the adjacent dorsal surface of the ascending colon. [Illustration: FIG. 559.--Human infant. Ileo-colic junction and caecum; dorsal view. Retro-colic appendix, adherent to the free dorsal serous surface of the large intestine, with intermediate extraperitoneal segment. (Columbia University Museum, No. 1638.)] [Illustration: FIG. 560.--Human adult. Ileo-colic junction and caecum; dorsal view. Appendix with intermediate non-peritoneal segment, while the proximal portion and the tip are covered by serous investment. (Columbia University Museum, No. 1615.)] C. ILEO-CAECAL FOLDS AND FOSSAE. Certain peritoneal folds, either mesenteric in character, _i. e._, containing blood vessels, or non-vascular, pass between the terminal ileum and the caecum and appendix, modifying in some instances very markedly the position and peritoneal relations of the structures. In considering the influence which these vascular mesenteric and non-vascular serous folds exert in producing further changes in the shape, position and relations of the human appendix it is necessary to remember that in the early embryonal stages these bands and folds of the peritoneum appear only slightly marked, but that they gain their importance and influence on the final adult configuration of the caecal pouch and appendix in the course of the further development of these structures.