Certain peritoneal folds, either mesenteric in character, i. e., containing blood vessels, or non-vascular, pass between the terminal ileum and the cæcum 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 cæcal pouch and appendix in the course of the further development of these structures.
For this reason the comparative study of the corresponding parts in other vertebrates, especially in certain mammalia, is of the utmost value, if we seek to explain and understand the derivation, significance and typical arrangement of these folds. We have seen that the cæcum as found in the large majority of mammalian forms is equivalent to the cæcum and appendix of the human subject and anthropoid apes; that in other words the vermiform appendix represents the distal segment of a cæcal pouch, originally uniform in caliber, which has remained undeveloped, while the proximal portion has progressed evenly with the general development of the alimentary canal to form the cæcum proper. We have seen that this tendency to retain the distal portion of the pouch in a rudimentary condition, i. e., the production of an appendage to the cæcum proper, is encountered in several of the lower forms, as certain Marsupials, Carnivores, Ungulates and Lemurs. The morphology of the ileo-cæcal folds is hence best understood by considering these structures as they appear in connection with the various cæcal types presented by the lower mammalia. Their arrangement and significance can here be readily made out. On the other hand, in studying these structures in the human appendix we are following lines which are already becoming indistinct on account of the rudimentary character of the organ, which we must regard as undergoing an exceedingly slow process of reduction, with a view to its ultimate elimination from the body. We have seen that the structural uncertainty impressed on cæcum and appendix by this evolutionary influence finds its expression in the wide range of variation in size and arrangement which these parts present. Necessarily, of course, this tendency to variation is shared, and even exhibited to a more marked degree, by what we can term the accessory structures connected with cæcum and appendix, viz., the mesenteric vascular and non-vascular serous folds passing to them from the ileum.
We can most profitably begin our consideration of these folds in a form in which they are preserved in their entire and original development, and then successively trace the changes leading up to the normal disposition in the human subject. Such a type is presented by the cæcum of Ateles ater, the black-handed spider monkey ([Figs. 444] and [445]). The cæcum of this animal presents a uniform crescentic curve, with the concavity directed upward and to the left, and the gradual diminution in the caliber of the pouch, from the ileo-colic junction to the apex, denotes the tendency to retain the distal segment in a rudimentary condition, foreshadowing the eventual formation of a vermiform appendix.
In the ventral view, with the terminal ileum lifted up, the following arrangement of folds passing between ileum and cæcum is noted ([Figs. 444] and [445]).
(a) Vascular Mesenteric Folds.—The peritoneal vascular folds, carrying the blood vessels to supply the cæcum, are two in number, a ventral (1) and dorsal (3). They are of nearly equal size and extent, passing from the ventral and dorsal aspect of the ileo-colic junction nearly to the apex of the cæcum. Each contains a branch of the ileo-colic artery, which forks in the ileo-colic mesentery, in the angle between ileum and large intestine. The ventral branch continues in the ventral mesenteric fold ([Fig. 445]) downward across the ventral surface of the ileo-colic junction to supply the ventral part of the cæcum, while the dorsal branch descends behind the ileo-colic junction, preserving a similar course in the dorsal mesenteric fold. The dorsal arterial branch is somewhat larger than the ventral and its distribution extends a little further down to the actual apex of the cæcum.
Fig. 561.—Human adult ileum with Meckel’s diverticulum. Ileo-diverticular serous fold and persistent omphalo-mesenteric artery. (Columbia University Museum, No. 1803.)
(b) Non-vascular Ileo-cæcal Serous Reduplication.—Between the two vascular mesenteric folds a third serous reduplication, carrying no blood vessels, is found passing between the ileum and cæcum. This fold begins, in the preparation from which the figure is taken, on the ileum opposite the attached mesenteric border, 2.7 cm. from the ileo-colic junction, and passes for exactly the same distance down on the adjacent left concave surface of the cæcum. It is placed a little nearer to the dorsal than to the ventral vascular fold, so that it passes, if the distance between the two vascular folds on the cæcum be divided into three parts, at the junction of the dorsal third with the ventral two thirds. The production of this intermediate non-vascular ileo-cæcal reduplication, which is of very constant occurrence in the mammalian series, is to be led back to the development of the cæcum. When the pouch protrudes from the smooth surface of the embryonic intestine opposite the mesenteric border, it extends backward along the future small intestine and lifts off the serous investment of the gut in the form of a small peritoneal plate filling the interval between itself and the adjacent ileum. A very perfect illustration of this process can be seen in the instance of Meckel’s diverticulum shown in Fig. 561. The proximal portion of the diverticulum is here still closely connected to the small intestine along which it extends, both being surrounded by the common visceral peritoneum. The distal part of the diverticulum has separated more completely from the intestine, and in so doing has drawn out the serous investment in the form of the triangular fold which is seen to pass between the free margin of the intestine and the adjacent surface of the pouch. The same process can be followed in its different stages in certain normal mammalian cæcal types.