206 DR. p. CHALMERS MITCHELL ON THE 



contents *. The caecum of the Rhinocei^os is a short cone taper- 

 ing to a point and regularly sacculated along three bands of 

 muscle, communicates with the hind-gut, of which it appears to 

 be the forward continuation, by a simple wide aperture, and its 

 normal contents are solid. 



The paired cseca are conical outgrowths, placed symmetrically 

 on the sides of the hind-gut. As shown in the figure (text- 

 fig. 13, II.), their cavities are widely continuous with that of the 

 hind-gut, and what I regard as the distal end of the ileum 

 enters the hind-gut exactly in the middle line between them. 

 In D. dorsaUs the ileum is lined by longitudinal lappets which 

 cease abruptly between the caeca, the lining membrane of these 

 being smooth, and that of the hind-gut studded with filiform 

 papillae. The contents of the cseca consist of faecal matter of the 

 same consistency and appearance as that in the hind-gut. Some 

 confusion has crept into the literature regarding the orientation 

 of the paired cseca. As in the case of the colic caeca of birds 

 and of mammals, Avhether there be a single caecum or a pair, 

 the caeca are the forward continuations of the hind-gut, and 

 their apices are directed forwards, parallel with the ileum, 

 towards the proximal extremity of the whole gut. As, however, 

 the tract lies folded within the body-cavity, in the undisturbed 

 condition, the portion of the gut to which the caeca are 

 attached ascends from the distal and ventral region of the 

 body towards the dorsal and anterior middle line, it may be said, 

 in the phrase of KauUa (Kaulla, 1830), that the caeca apice pelvein 

 speotant. The paired caeca, in fact, lie on the recurrent limb of 

 the pendant loop, the position in which the true caeca of all 

 mammals lie. This morphological position, which in my opinion 

 is sufiicient to identify the paired caeca of the Hyracoidea as the 

 homologue of the true cfeca of mammals, is quite apparent if the 

 various diagrams I have given in this memoir, and in my earlier 

 memoir, be compared. But the homology is equally plain from 

 another consideration. When the abdomen of any mammal is 

 opened, the caecum, if it exist, is found with its attachment to 

 the gut towards the right side of the body, more anteriorly or 

 posteriorly placed according to its place on the recurrent limb of 

 the original pendant loop. If the caecum be very large, and 

 especially when it is long and coiled, it may extend towards the 

 left side of the body, reaching well across the middle line. If it 

 be very small, its position on the right side is obvious. As a 

 supposed resemblance between the unpaired caecum of Hyrax 

 and the noi-mal mammalian caecum of the Rhinoceros has been 

 alleged against the homology I make, I may refer to the figures 

 of the undisturbed abdominal viscera of the Rhinoceros given 

 by Garrod (Garrod, 1873, fig. 5) (Beddard & Treves, 1887, 



* From observations on a living Syrax, whicli was in my possession for nearly 

 eighteen months, I infer that the contents of the intestines may pass directly from 

 the aperture of entrance to the aperture of exit of the accessory caecum, and that the 

 latter gradually tills with a fluid and is discharged at infrequent intervals (usually 

 about fortnightly), apart from the normal daily defaecation of solid faeces. 



