524 DR. P. CHALMERS MITCHELL ON THE 



the functions of the different parts of the intestinal tract is still necessary before the 

 extent to which adaptation has modified the caecum can be properly estimated, but, so far 

 as I can judge from the anatomical data assembled here, it seems a reasonable inference 

 that ancestral history is at least as important as adaptation to present function. 



i 



Secondary Relations between the Proximal and Distal Portions 



of the Intestinal Tract. 



In my memoir on the intestinal tract in Birds, I called attention to a peculiar 

 relation between the proximal and distal portions of the gut. In the natural condition 

 the duodenum, which in Birds is almost invariably a separate, distinct loop, lies ventrally 

 to the rest of the gut. Cuvier called attention to the close interconnection between 

 the duodenum and a distal loop of the gut, terming the latter loop the colon. The 

 term is confusing, because the Mammalian colon is a region distad of the caecum or 

 caeca, whereas in Birds the portion of the gut which comes into intimate relation with 

 the duodenum is proximad of the caeca, and is indeed the most distal part of Meckel's 

 tract. In Birds, Meckel's tract displays a strong tendency to be divided into distinct 

 minor loops, and it is the most distally placed of these loops that enters into relations 

 with the duodenum, and to which I therefore gave the name " Supra-duodenal Loop." 

 In the more primitive types of Birds this interconnection is but little marked and has 

 but little importance. In the higher members of each Avian series, however, there is 

 a tendency for the interconnection to be very highly developed. In the Passeres, for 

 instance, the supra-duodenal loop is very accurately moulded on the duodenum, these 

 two regions of the gut lying in close application, with a partial fusion of their 

 mesenteries, and with a complex interchange of blood-vessels and nerves. The 

 possibility of this interconnection having been formed is due to the mode of growth of 

 the gut, the greater part of the length in the adult being formed from only a very 

 narrow portion of the primitive straight antero-posterior tube, so that loops which may 

 be separated by a long stretch of gut in the adult may come into close contact as they 

 hang suspended from a very narrow attachment to the dorsal wall of the cavity. But 

 although the possibility of developing such interconnections exists for many regions 

 of the fore-gut in Birds, the interconnection has been developed so constantly and 

 progressively between the duodenum and the supra-duodenal loop, and so slightly and 

 so rarely in any other region, that there is a strong suggestion of some functional 

 adaptation. In Mammals, as in Birds, the greater part of the length of the gut has 

 similarly arisen as the outgrowth of only a very small part of the primitive straight gut, 

 and there is thus a similar opportunity for the establishment of interconnections being 

 formed between the closely bunched loops. In Mammals, as in Birds, the actual 

 interconnection is limited to a definite region. The region, however, is different in 

 the two cases. In Mammals the duodenum is very seldom a definite loop ; Meckel's 



