INTESTINAL TRACT OF MAMMALS. 243 



which I gave a long account (Mitchell, 1905, p. 524) was that in 

 which blood-vessels belonging to one region of the gut supply 

 another region with which it may be in contact, although 

 morphologically remote. In Birds the folding of the gut brings 

 the distal portion of Meckel's tract in close contact with the 

 duodenum, and it frequently comes about that branches of 

 the duodenal blood-vessels may form the main supply of the 

 portion of Meckel's tract just proximal to the caeca, and may 

 have to be severed before the whole gul7 can be unfolded. In 

 Mammals the connection, when it exists, links the colic region to 

 the anterior part of the gut. I wish to modify the table I gave 

 only by omitting Ornithorhynchtis ; from examination of another 

 example, I am far from certain as to the existence of a true 

 " short-circuiting " blood-vessel, and the point could be settled 

 only by exa,mination of fresh injected material. The cases, then, 

 in which this peculiar condition of the blood-vessels certainly 

 exists are the Traguloidea, Tylopoda, Pecora, Rodents, Lemurs, 

 and Simise. If one considers it, it is a curious circumstance that 

 in the development of man a bi^anch of the superior mesenteric 

 artery should leave its normal course and thrust itself out 

 to reach the transverse colon. Instead of explaining this as 

 an instance of some marvellous coordinating vitalistic power, I 

 prefer to think that it is a legac}^ from the past, and that the 

 ancestors of the Simise had a more complex colon with loops 

 pressed against the mesentery of Meckel's tract, as occurs in 

 some of the Lemurs. In this connection it is interesting to note 

 that Klaatsch found a Lemur-like stage of the colon in the 

 embryo of HcqDcde (Klaatsch, 1892, p. 671, fig. 12, cited by 

 Beddard, 1908, p. 598). 



There are also connections of a more mechanical kind between 

 different portions of the gut. These are the various "ligaments" 

 and attachments to which I have frequently referred in this 

 communication. They were not included in the table in TO-y 

 paper of 1905. ISTotwithstanding the elaborate work of Klaatsch 

 (Klaatsch, 1892), and Dr. Beddard's later discussion (chiefly 

 Beddard, 1908, p. 568 et sequiUir), I cannot form a clear con- 

 ception of the distribution of these structyres among Mammals, 

 and I have not myself made a connected investigation of them. 



Loops of the Hind-gut. — I have already drawn a contrast 

 between the gut-patterns of Birds and Mammals, depending on 

 the broad fact that, even when allowance has been made for the 

 homoplastic modifications associated with diet (Mitchell, 1905, 

 p. 526), in Birds Meckel's tract and in Mammals the hind-gut 

 tend to display specialized subsidiary loops of systematic im- 

 portance. In Birds, however, the loops of Meckel's tract have 

 reached a high degree of stability, so that they vary little within 

 well-defined systematic groups, whereas in Mammals the loops of 

 the hind-gut vary much more within narrow systematic limits, 

 as if thev were in much closer relation with habit or diet. Th©' 



16* 



