476 DR. P. CHALMERS MITCHELL ON THE 



of turns. The supra-Meckelian loop is also very long, and is so closely applied to the 

 course of Meckel's tract that it is difficult to dissect it off. The rectal portion varies 

 in length, being relatively longest, most capacious, and most convoluted in the 

 Giraffe. 



The portal system presents a number of minor variations which seem not to be 

 distributed in accordance with the systematic divisions of the animals in question. 

 The anterior mesenteric vein always follows the course of Meckel's tract, closely 

 receiving more or less regularly arranged minor factors from the subsidiary loops. 

 Where, as in the Giraffe, the course of the tract nearly completes a circle, a large 

 vessel may run across the diameter of the mesentery. The caecum is drained by a vein 

 which joins the anterior mesenteric usually some distance from the edge of the 

 mesentery. The spiral loop is drained by a large vessel which sometimes joins 

 the csecal vein, sometimes enters the anterior mesenteric vein proximad of the csecal 

 vein. The posterior mesenteric or rectal vein joins the anterior mesenteric vein very 

 near the edge of the mesentery. 



The Ruminant Artiodactyles, then ? display a pattern peculiar to the group, and 

 characterised by the enormous length, special modification, and arrangement of the 

 hind-gut. In all, the hind-gut displays three well-marked regions : a spiral loop 

 simplest in Tragulus, in the others forming a closely-coiled, flat, watchspring-like 

 arrangement, folded over on the mesentery that supports Meckel's tract ; a supra- 

 Meckelian fold, which in the characteristic and most specialised cases is stretched round 

 Meckel's tract just at the line where the minor folds leave the mesentery, and which 

 is drained by branches from the vessels of Meckel's tract ; and a rectal poi'tion, the 

 degree of convolution of which varies nearly directly with the size of the animal. 



The Non-Ruminant Artiodactyles display a pattern fundamentally similar to, but less 

 complicated than, that of the Ruminant forms. Meckel's tract is almost identical in 

 its disposition. The spiral coil of the hind-gut is present and is very large, but its 

 calibre is wider in proportion to its length and the coiling is not so flat. There is no 

 more than a trace of the supra-Meckelian fold, so that the hind-gut, although long, is 

 less differentiated. 



Order PERISSODACTYLA. 



Family Tapirid/e. Tapirus americanus (fig. 23). 



In the Tapir the duodenum is a long and well-differentiated narrow loop. Meckel's 

 tract is comparatively short, and consists of a small number of relatively large loops 

 suspended at the periphery of a nearly circular expanse of mesentery. The caecum is 

 long, very capacious, and irregularly sacculated. The hind-gut is divided into two 

 well-marked regions. The first region consists of an enormously long and capacious 

 colic loop, the proximal and distal limbs of which lie close together, and of a relatively 

 long and nearly straight rectum. The anterior mesenteric vein curves round the 



