INTESTINAL TRACT OE MAMMALS. 441 



as the Hind-gut. It corresponds with a much larger part of the primitive straight gut 

 than the duodenum and Meckel's tract together, and its proximal portion, in conse- 

 quence, is extremely close to the origin of the duodenum. In the vast majority of 

 Birds, this portion is relatively extremely short, often being only from one-tenth to 

 about one-thirtieth of the whole length of the gut. Birds, in fact, have anticipated 

 the suggestion of Metchnikoff, and have got rid of the large intestines and rectum 

 almost entirely. A certain number of primitive forms, such as Struthious Birds, alone 

 have retained them. Mammals may be contrasted with Birds as a group in which the 

 hind-gut is always relatively long, sometimes extremely long, amounting to almost 

 one-half of the total length of the intestinal tract. The Ostrich is the only bird that 

 can be described as mammalian in the character of its hind-gut. Moreover, in 

 Mammals the hind-gut shows a strong tendency to differentiation into regions the 

 characters of which are of systematic importance. The first region is the colon or 

 large intestines. This region presents very varied modifications, and a general 

 feature of these is a tendency to close relationship with the duodenum, in position 

 and sometimes by short-circuiting blood-vessels. In fact this region corresponds with 

 the supra-duodenal loop of Birds in relationships aud possibly in function, although it 

 is differentiated from a region of the gut morphologically distinct. Like the whole of 

 Meckel's tract, it is an outgrowth of a relatively very short portion of the primitive 

 dorsally placed straight gut. The second region of the hind-gut runs from the distal 

 end of the colon to the anus. It may be called the rectum, and although it is some- 

 times lengthened, it is typically not very much longer than the portion of the primitive 

 straight gut which it represents. It is most dorsally placed in the body-cavity, and its 

 proximal end is primitively very near to the duodenum. In some Mammals there is 

 no differentiation of the hind-gut into colon and rectum ; in others the differentiation 

 is extreme. A final contrast between Mammals and Birds (in the morphology of the 

 gut) may be stated as follows : — In Birds the portion of the gut known as Meckel's 

 tract in the course of its lengthening tends to fall into subsidiary loops, the number, 

 position, and patterns of which are of systematic value, whilst the hind-gut tends to 

 remain simple ; in Mammals, Meckel's tract, even although greatly lengthened, does 

 not fall into conspicuously different patterns, but remains a simple coil or series of coils 

 at the periphery of a circular expanse of mesentery, the hind-gut tends to lengthen, 

 and the number, position, and patterns of the subsidiary loops thus formed are of 

 systematic value. 



