Fig. 562.—Human adult. Ileum with Meckel’s diverticulum, 131.5 cm. from ileo-colic junction; a distinct vascular fold is prolonged from the ileal mesentery to the margin of the diverticulum. (Columbia University Museum, No. 1849.)
In this connection it may be noted that the production of the cæcal vascular folds and their relation to the mesentery is also very perfectly illustrated in some forms of Meckel’s diverticulum. Thus in the preparation shown in Fig. 562, a broad triangular serous fold passes from the ileal mesentery to the margin of the diverticulum, carrying the blood vessels which supply the pouch. If the section of the intestine to the left of the figure is regarded as representing the terminal ileum, that to the right the colon, and the diverticulum the cæcal pouch, the formation of the fold and its relation to the mesentery, blood vessels and intestine will correspond closely to the ileo-cæcal vascular folds.
[Fig. 350] shows the ileo-colic junction and cæcum of Halmaturus derbyanus, the rock kangaroo. The cæcum here extends backwards along the free border of the ileum to which it is closely bound by the common investing visceral peritoneum for the greater part of its extent. In another marsupial form, a small species of opossum from Trinidad ([Fig. 349]), the cæcum has separated itself more completely from the adjacent small intestine—thus drawing out the peritoneum into a narrow connecting fold. Finally, in the Virginia opossum ([Fig. 348]), the ileum has attained the usual position at right angles to cæcum and colon. The former pouch is separated from the small intestine by a considerable interval and the angle between the two is filled out by a well-developed triangular serous fold, connecting the free margin of the terminal ileum and the adjacent left border of the cæcum.
This is the “intermediate non-vascular” ileo-cæcal fold.
Passing now from the condition presented by Ateles, with three fully developed and distinct ileo-cæcal folds, to the next stage leading up to the normal human arrangement, we find the same illustrated in the cæcum of another new-world monkey, Mycetes fuscus, the brown howler monkey, shown in the ventral and dorsal views in [Figs. 449] and [450]. The ventral vascular fold ([Fig. 449], 1) is still well developed, the contained ventral branch of the ileo-colic artery descending over the ventral wall of the ileo-colic junction and cæcum and supplying both. The dorsal vascular fold ([Fig. 450], 2), on the other hand, is nearly completely fused with the intermediate non-vascular reduplication ([Figs. 449] and [450], 3), the approximation between these structures exhibited by Ateles having in Mycetes reached the point of actual union, so that the larger dorsal branch of the ileo-cæcal artery descends to the apex of the cæcum in the following manner: The main post-cæcal artery passes over the dorsal surface of the ileo-colic junction included in a short serous fold which corresponds to the dorsal vascular fold of Ateles. Beyond the lower border of the ileo-colic junction this fold fuses with the intermediate non-vascular fold, one arterial branch descending along the line of attachment of this fold to the cæcum, the other distributed over the dorsal surface of the pouch.
A third type, also taken from the lower Primates, is presented by the cæcum of a cynomorphous monkey, Cercopithecus sabæus, the African green monkey, shown in [Fig. 432], in the ventral and left aspect with the terminal ileum lifted up. The cæcum of this animal is comparatively short, somewhat conical, terminating in a blunt apex. The vascular supply is arranged on the same type as in Ateles and Mycetes, i. e., a trunk of the ileo-colic artery divides at the ileo-colic notch, one branch descending ventrad, the other dorsad of the ileo-colic junction. The slightly larger size of the dorsal vessel, noted in Ateles and Mycetes, has been increased in Cercopithecus until the ventral artery (1) supplies merely the front of the ileo-colic junction and the upper part of the adjoining ventral wall of the cæcum, while the larger dorsal vessel (2) descends behind the ileo-colic junction, supplying the same and the entire dorsal and apical portions of the cæcum. The relation of these cæcal arteries to the peritoneum is moreover different from that encountered in Ateles. In place of running in distinct mesenteric folds, as in the latter species, the vessels pass close to the surface of the intestine, merely covered and partly surrounded by slightly redundant visceral peritoneum containing numerous pads of epiploic fat, which bead the course of the vessels at regular intervals. Between the two arteries the intermediate non-vascular fold (2) is seen, presenting much the same arrangement as in Ateles and passing between the left border of the cæcum and the adjacent margin of the ileum, nearer to the dorsal larger than to the ventral smaller cæcal artery.
We have, therefore, in the three types just considered, the following variations in the arrangement of the vascular and non-vascular folds:
| 1. | (a) | Ventral and dorsal vascular folds distinct and free. Ventral and dorsal cæcal arteries of nearly equal size. | Ateles. | ||
| (b) | Intermediate non-vascular fold free on both surfaces, placed nearer to the dorsal than to the ventral vascular fold. | ||||
| 2. | (a) | Ventral vascular fold distinct. Ventral cæcal artery somewhat further reduced in size. Dorsal vascular fold distinct only over the dorsal surface of the ileo-colic junction. At the lower border of the ileo-colic junction the dorsal vascular fold fuses with the intermediate non-vascular fold. | Mycetes. | ||
| (b) | Intermediate non-vascular fold free only on ventral surface, the dorsal surface below the ileo-colic junction being fused with the dorsal vascular fold. | ||||
| 3. | (a) | Dorsal and ventral vascular folds reduced. Dorsal artery much larger than ventral. | Cercopithecus. | ||
| (b) | Intermediate non-vascular fold well developed, free on both surfaces. | ||||