IXTESTINAL TRACT OF MAMMALS. 221 



growth to the left an ansa sinistra, then both tlie spinal loop and 

 the supra- meckelian fold of Pecora, Tjdopoda, and Traguloidea 

 may be taken to represent ansce dextrce. 



Farther work, and the consideration of the points raised by 

 writei's who have followed me, have not given me any reason to 

 modify the general summary I gave in 1905 (Mitchell, 1905, 

 p. 476): — "The Ruminant Artiodactyles display a pattern 

 peculiai' to the group, and chai-acterised by the enormous length, 

 special modification, and ai-rangement of the hind-gut. In all, 

 the hind-gut displays three well-marked regions : a spiral loop 

 simjoler in Tragulus, in"' (most of) "the others forming a, closely- 

 coiled, flat, watch-spring 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 tlie minor folds leave 

 the mesentery, and which is di'ained by branches from the vessels 

 of Meckel's tract ; and a rectal portion, the degree of convolution 

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



"The non- ruminant Artiodactyles display a pattern funda- 

 mentally similar to, but less complicated than, that of ruminant 

 forms. Meckel's tract is almost identical in its disposition. 

 The spiral coil of the hind-gut" (usually, not in the Hippo- 

 potamus) " is present and is very large, but its calibre is widei- 

 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. (Text-figure 20.) 



I have little to add to the account T have already given 

 (Mitchell, 1905, p. 476, figs. 23, 24, 25) of the gut-pattern of 

 the Rhinoceros, Tapirs, and Equidse. For convenience, I repeat 

 as text-fig. 20 the figure I have already given (Mitchell, 1905, 

 fig. 25) of the gut-pattern of Eqims (jranti. I have added at 

 M. the portion of primitive mesentery between the caecum a,nd 

 the ileum, and at XX have marked the line along wdiich the 

 cfecum is tied by adventitious fibres to the colic loop. I have 

 verified these points on the domestic horse, as no example of a 

 zebra was available. In the thi'ee families the pa.ttern is quite 

 definite and I'emarkably uniform. The duodenum is a distinct 

 loop, Meckel's tract is relatively short and compact, the caecum 

 is very capacious, but relatively smaller in the Rhinoceros than 

 in the others, and situated rather high up on the recurrent limb 

 of the pendant loop. It is greatly exceeded in capacity by an 

 enormous narrow loop, each limb of which is large in calibre, 

 formed as an outgrowth of the distal jjortion of the recurrent limb. 

 The hind-gut then bends round to form the relatively simple 

 rectum. The caecum is connected with the distal extremity of 

 the ileum by a short doul)le fold of mesenteiy, the usual remnant 

 of the jDrimitive mesentery which is found in this situation in 



