462 DR. P. CHALMERS MITCHELL ON THE 



subdivided into a distinct colic loop (C.L.) and a long straight rectum. The portal 

 system consists of an anterior mesenteric vein which arises at the periphery of Meckel's 

 tract, in a fashion similar to the most common disposition in Birds ; it runs straight 

 through the suspensory mesentery of Meckel's tract, receiving tributaries on either 

 side, one of which is the duodenal A'ein. There is a middle mesenteric vein draining 

 the distal portion of Meckel's tract and the paired caeca, and this is joined posteriorly 

 by a colic vein and a large posterior mesenteric or rectal vein. 



The difficulty in the interpretation of this intestinal tract is the presence on Meckel's 

 tract of a large expansion, which I propose to call the accessory caecum, and which is 

 marked A.C. in fig. 14. This structure has been described and figured frequently, 

 and, so far as I am aware, has always been set down as the normal Mammalian caecum, 

 the pair lower down being regarded as appendages peculiar to Hyrax. I think that a 

 comparison of the figure of the tract of Hyrax with the figures of the intestinal tracts 

 of the other Mammals described in this memoir is enough to suggest at once that the 

 paired caeca are the representatives of the normal caecum of Mammals. Their relations 

 to the rest of the alimentary canal, and, in addition, the evidence that I have been able 

 to adduce from the conditions in Marsupials, Sloths, and Anteaters as to a paired 

 condition of the caeca being a primitive mammalian character, make these organs in 

 Hyrax fall into line with the general disposition of the mammalian intestinal tract. 

 Moreover, apart from the abnormality of the position of the unpaired caecum, were it 

 to be regarded as the representative of the normal mammalian caecum, there is 

 nothing about its structm - e or relation to its entrance and exit making it similar to 

 any Mammalian caecum with which I am acquainted. The character of the intestinal 

 canal before and after the accessory caecum has been referred to by George (8), who, 

 in his elaborate monograph on Hyrax, gave a figure of the stomach, part of the 

 alimentary tract, and the unpaired caecum seen from the outside and after being 

 opened. Following Cuvier, he took for granted that the unpaired caecum was the 

 representative of the normal mammalian caecum, and noticed, as a remarkable circum- 

 stance, that the paired caeca resembled those of Birds. He states correctly that the 

 small intestines, both anterior and posterior to the unpaired caecum, have the interior 

 surface increased by a series of folds of the mucous membrane, these folds, however, 

 unlike the normal valvulae conniventes, being longitudinally directed. On the other 

 hand, after the paired caeca, the intestinal tract becomes wider in calibre and with a 

 smoother internal surface. Dr. Seligmann (the Society's pathologist) was kind enough 

 to prepare sections for me from pieces selected from the gut proximal to the unpaired 

 caecum, between the unpaired and the paired caeca, and distal to the paired caeca. 

 Although the material was somewhat macerated, not having been preserved for 

 ■microscopical purposes, it was sufficiently good to show the general nature of the wall 

 of the gut in these different regions. In the anterior portion the cavity was con- 

 siderably reduced by the folds in the walls, and by closely set extremely long villi, as, 



