INTESTINAL TEACT OE MAMMALS. 439 



a series of folds. The mesentery grows 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 outgrowing loops 

 invade each other's localities, with occasional " short-circuiting " of their blood-vessels, 

 disturb the primitive simplicity. Three definite regions, however, require separate 

 treatment. The first region is the duodenum. In the vast majority of Birds, and 

 in many Reptiles and Amphibia, the portion of the gut immediately distad of the 

 stomach grows out into a long and narrow loop, 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 many of the simpler Mammals (vide the systematic 

 part of this memoir, infra, p. 442) the duodenum is a simple fold, but in others it is 

 thrown into a large series of minor loops which pass insensibly into the next portion 

 of the gut. Taking Mammals generally, they may be said to be characterised by a 

 tendency to less definite separation of the duodenum as a specialised portion of the gut. 

 The second portion is what I have termed Meckel's Tract (Mitchell, 15). It consists 

 of the part generally known as the small intestines and stretches from the distal end 

 of the duodenum to the caecum or cseca. 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 narrow part of the original straight gut, corresponding to not more than 

 two or three primitive somites of the body. This narrow area grows out to form the 

 greater part of what is known as the " pendent loop" in mammalian embryology. Its 

 anterior, or proximal, end is close to the approximated proximal and distal ends of the 

 duodenal loop, and its distal end. passes into the rectal portiou. In the embryos of all 

 Birds and Mammals the median point of Meckel's tract, the part of the loop which has 

 grown out furthest from the dorsal mesentery, is marked by the diverticulum caecum 

 vitelli, the primitive connection of the cavity of the gut with the narrowing stalk of 

 yolk-sac. Naturally, in Birds and Reptiles, where the yolk-sac is of great functional 

 value, this diverticulum is large and important. The main factor of the portal vein, 

 that known as the anterior mesenteric division, runs from this to the dorsal edge 

 of the mesentery near the duodenum, receiving on its way tributaries from the whole 

 course of the small intestines. In perhaps a majority of the families of Birds a 

 remnant of this diverticulum persists throughout life, and in some cases even blossoms 

 out into a second or deuteroblastic growth and activity. In Mammals, no doubt in 

 association with the reduction of the yolk-sac, this diverticulum, which is known as 

 Meckel's diverticulum, has less importance. It is seldom the starting-point of the 

 largest tributary of the middle mesenteric vein, and its presence in the adult must be 

 extremely rare. Birmingham (2), it is true, states that it occurs in 2*2 per cent, of 

 adult human subjects. I myself have examined the intestinal tract of over two hundred 

 Mammals, including many examples of several species, and although I have always 

 carefully looked for it, have found it in no single instance. It is not present even iu 



3n -1 



