mesentery 
Fig. 5.—Intestinal Tract of Canis vulpes. S, cut end of duodenum; C, caecum; R, cut end of rectum.grew out with these folds, but the presence of adjacent organs, the disturbance due to the outgrowth of the liver, and the secondary relations brought about between different portions of the gut, as the out-growing loops invaded each other’s localities, disturbed the primitive simplicity. Three definite regions of outgrowth, however, became conspicuous and are to be recognized in the actual disposition of the gut in existing birds and mammals. The first of these is the duodenum. In the vast majority of birds, and in some of the simpler mammals, the portion of the gut immediately distal of the stomach grows out into a long and narrow loop (fig. 4, d), the proximal and distal ends of which are close together, whilst the loop itself may remain long and narrow, or may develop minor loops on its course. In mammals generally, however, the duodenum is complex and is not so sharply marked off from the distal portion of the gut as in birds. The second portion is Meckel’s tract. It consists of the part generally known as the small intestines, the jejunum and ileum of human anatomy, and
stretches from the distal end of the duodenum to the caecum or caeca. It is the chief absorbing portion of the gut, and in nearly all birds and mammals is the longest portion. It represents, however, only a very small part of the primitive straight gut, corresponding to not more than two or three somites of the embryo. This narrow portion grows out to form the greater part of what is called the pendent loop in mammalian embryology. Its anterior or proximal end lies close to the approximated
proximal and distal ends of the duodenal loop, whilst its distal end passes into the hind-gut at the colic caecum or caeca. In the embryos of all birds and mammals, the median point of Meckel’s tract, the part of the loop which has grown out farthest from the dorsal edge of the mesentery, is marked by the diverticulum caecum vitelli, the primitive connexion of the cavity of the gut with the narrowing stalk of the yolk-sac (fig. 4, y.) Naturally, in birds where the yolk-sac is of great functional importance this diverticulum is large, and in a majority of the families of birds persists throughout life, forming a convenient point of orientation. In mammals, no doubt in association with the functional reduction of the yolk-sac, this diverticulum, which is known as Meckel’s diverticulum, has less importance, and whilst it has been observed in a small percentage of adult human subjects has not been recognized in the adult condition of any lower Mammalia.
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| Fig. 6.—Intestinal Tract of Macropus bennetti. S, cut end of duodenum; R, cut end of rectum; C, caecum; C2, accessory caecum; C.L, colic loop of hind-gut. |
Fig. 7.—Intestinal Tract of Tapir. S, cut end of duodenum; R, cut end of rectum; C, caecum; CL, colon. |
In birds, Meckel’s tract falls into minor folds or loops, the disposition of which forms a series of patterns remarkably different in appearance and characteristic of different groups. In fig. 4 an extremely primitive type is represented. In mammals Meckel’s tract remains much more uniform; it may be short, or increase enormously in length, but in either case it falls into a fairly symmetrical shape, suspended at the circumference of a nearly circular expanse of mesentery. Where it is short it is thrown into very simple minor loops (figs. 5, 6 and 7); where it is long, these minor loops form a convoluted mass (figs. 8 and 9).
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| Fig. 8.—Intestinal Tract of Giraffe. S, cut end of duodenum; R, cut end of rectum; C, caecum; P.C.L, post-caecal loop; S.P, spiral loop; SF, third loop of hind-gut. |
The third portion of the gut should be termed the hind-gut and lies between the caecum or caeca and the anus, corresponding to the large intestines, colon and rectum of human anatomy. It is formed from a much larger portion of the primitive straight gut than the duodenum and Meckel’s tract together, and its proximal portion, in consequence, lies very close to the origin of the duodenum. In the vast majority of birds, the hind-gut in the adult is relatively extremely short, often being only from one-eighth to one-thirtieth of the whole length of the gut. A certain number of primitive birds, however, have retained a relatively long condition of the hind-gut (fig. 4), the greatest relative length occurring in struthious birds, and particularly in the ostrich, where the hind-gut exceeds in length the duodenum and Meckel’s tract together. Mammals may be contrasted with birds as a group in which the hind-gut is always relatively long, sometimes extremely long, and in which, moreover, there is a strong tendency to differentiation of the hind-gut into regions
