INTESTINAL TEACT OF MAMMALS. 515 



As a group, the Simiae display a quite definite pattern of the intestinal tract. 

 The duodenum and Meckel's tract together form a series of loops which differ from 

 group to group in their relative complexity, arranged round about three-quarters of 

 the circular outgrowth of mesentery. The caecum is always present and appears to 

 have been originally capacious and of nearly equal calibre throughout its length ; but 

 it is in process of shortening throughout the group, being, as a rule, shorter in the 

 Old World Monkeys than in the New World Monkeys and the Anthropoid Apes 

 (if in the latter case the vermiform appendix be reckoned with the caecum). The 

 state of the case may be put in another way. The originally long capacious caecum 

 of the Simiae is retained by the greater number of the Platyrrhine Apes ; in the 

 Catarrhine Apes, except the Hylobatidae and the Anthropomorphae, it tends to become 

 shorter without the formation of a vermiform appendix. In the two last-named groups 

 its proximal portion has remained capacious, but the greater part of its original length 

 has been transformed without shortening into the thick-walled vermiform appendix. 



Family HominiDjE. 



Human anatomy is so well known that it is unnecessary to do more than refer to 

 the essential similarity between the pattern of the human intestinal tract and that of 

 the Anthropomorphae. Probably primitive man was of omnivorous, with a tendency 

 to carnivorous, diet, but the result has not yet been any marked adaptive change in the 

 character of the gut as compared with that of the Anthropoid Apes, although in 

 the latter the diet is omnivorous with the strongest leaning towards the vegetable side. 



Homology of the Mammaliax CLecum. 

 Notwithstanding the existence of a voluminous literature on the caecum, 1 cannot find 

 that any attempt has been made to institute a comprehensive comparison of the caeca 

 in Mammals and Birds or to decide as to whether or no these structures are to be 

 regarded as homologous. 1 have no doubt myself but that the structures in the two 

 groups are homologous, and that any anatomist Avho had the opportunity of studying 

 them in a large series of Birds and of Mammals would come to the same conclusion. 

 The anatomical evidence for the conclusion may be examined conveniently under 

 three heads — the position of the caeca in the course of the intestinal tract, the occasional 

 occurrence of paired caeca in Mammals, and the relations of the adjacent extremities of 

 the ileum, caecum, and colon in many Mammals. 



Position of the Caeca. 



In the vasl majority of birds the hind-gut is relatively very short, and the paired 

 caeca, accordingly, lie relatively close to the cloaca. In a Fowl and Pheasant, for 

 instance, where the length of the intestinal tract from the stomach to the cloaca may 



