532 DR. P. CHALMERS MITCHELL ON THE 



in extreme cases the proximal colic region and the distal part of Meckel's tract. This 

 disposition of the increased length of gut, consequent on adaptation to vegetable diet, 

 is peculiar to Rodents. 



In most of the living Insectivora the pattern of the intestinal tract is of reptilian 

 simplicity, consisting of a series of very simple short loops suspended by a continuous 

 mesentery, without caeca or differentiation into regions. There is evidence, however, 

 that this simplicity is secondary. Some genera have a caecum, and in some there is a 

 differentiation into fore-gut and hind-gut, whilst the latter again can be divided into 

 colic loop and caecum. On the evidence, we can say only that the Insectivora have 

 departed from the ancestral mammalian type by reduction of the gut. 



The Chiroptera display the same- conditions as the Insectivora, save that the 

 reduction has gone further, and the traces of former complexity are rarer. 



The Galeopithecidae, so far as I can gain information from other authors, are much 

 less reduced than the Insectivora or Chiroptera, and display at least the primitive 

 mammalian differentiation of the gut, recalling the Prosimiae rather than the 

 Insectivora. 



The Carnivora show a reduction from the pattern of the primitive mammalian 

 intestinal tract so far as the hind-gut and caecum are concerned, whilst Meckel's tract 

 is frequently elongated. The varying conditions of the caecum from complete absence 

 to moderate length, and the indications of former greater length of the hind-gut, are 

 scattered throughout the group in the familiar mode of degenerate structures. In a 

 broad way, however, it may be said that those Carnivora which are most specialised 

 in other directions are most degenerate as regards the caecum and hind-gut. 



The intestinal tract of the Prosimiae is not far removed from the primitive 

 mammalian pattern, Meckel's tract remaining simple and not relatively long, whilst 

 the hind-gut, in nearly all the members of the group, is very long and capacious, and 

 divided into colic loop and rectum. The colic loop tends to grow out in a single, long 

 and narrow loop, sometimes simple, sometimes with subsidiary folds on its course, 

 suggesting, perhaps, a remote connection with the Ungulate stock. 



The Simiae, with which, for the present purpose, the Hominidae may be reckoned, 

 display a type of intestinal tract which in many respects is simpler than that of the 

 Prosimiae and would appear to have been independently derived from the primitive 

 mammalian pattern. Meckel's tract varies, but tends to lengthen and to be arranged 

 in a closely set series of long minor loops. Traces of a primitive paired condition of 

 the caeca are frequent, but the normal single caecum is not so long as that found in 

 the Prosimiae. The hind-gut is usually relatively long and capacious, but shows no 

 signs of the formation of the long narrow colic loop found in the Prosimiae. In the 

 simpler cases, it is composed of a series of minor loops suspended by a mesentery which 

 is practically continuous with the Meckelian mesentery, so that the configuration of the 

 whole intestinal tract (e. g. fig. 44, p. 512) recalls that of some of the simplest Mammalian 



