INTESTINAL TRACT OF MAMMALS. 519 



conclusion that the same structures are involved in each case. The drawing labelled 

 with the Eoman numeral I represents the junction of the ileum, colon, and symmetri- 

 cally paired caeca in Dasypus minutus. A "window" was cut in the wall of the gut 

 to show the relation of the posterior end of the ileum with its projecting, tubular 

 orifice placed between the orifices of the caeca. The apparent discontinuity between 

 the ileum and the colon is seen to be a simple consequence of the outgrowth of the 

 paired caeca. The drawing labelled II in the same figure (47), which represents the 

 condition in Petaurus sciurevs, is obviously identical morphologically with the condition 

 in Dasypus, with the difference that the caecum marked C2 is much reduced. In the 

 Gazelle (fig. 47, III) the condition is again obviously repeated, but one caecum is still 

 further reduced, whilst the aperture of the ileum instead of being projecting and tubular 

 is a slit between two valvular flaps, the position of which is represented by dotted 

 lines. 



Relation between the Ileum, Ccecum, and Colon. 

 The very peculiar valvular arrangements that occur where the ileum, caecum, and colon 

 meet have been described and studied in a number of cases, but, so far as I am aware, 

 the attention of observers has been directed chiefly to the functional peculiarities of this 

 region. The structures are generally adapted to a greater or less extent to allow 

 free exit of the contents of the ileum into the lower bowel and free communication 

 between ihe caecum anel colon without regurgitation of the contents of the latter organs 

 into the ileum. A further adaptive series of modifications varies with the degree to 

 which the caecum is modified as a glandular rather than an absorbing or digesting organ. 

 Behind the adaptive modifications, however, lies the question of the morphological 

 material from which these modifications have been produced. The latter vary very much 

 from genus to genus, sometimes from species to species, but throughout the series there 

 is the common feature that they are easy to understand if it be supposed that originally 

 the ileum opened into the colon between symmetrically placed caeca, and difficult to 

 understand if the traditional view be accepted that the mammalian caecum is an unpaired 

 structure. In fig. 48, p. 520, the condition is represented as it occurs in two animals 

 very far removed in the mammalian series, fig. 48 I being drawn from Babirussa and 

 fig. 48 II from Mycetes beelzebul. In both cases the colon, cut across at II, widens 

 considerably before it joins the dilated conical caecum C. Windows have been cut in 

 the wall of the gut to show the relation of the tubular free edge of the ileum, which 

 projects into the expanded end of the colon precisely as if a hole had been cut in the 

 wall of the latter and the tubular end of the ileum pushed through into the cavity. 

 Just anterior to this in each case is the aperture of the caecum with a raised oval lip. 

 If it be supposed that in the region of the colon vertically under the letters LA. 

 in the drawings there was formerly a second caecum, the anatomical relations at once 

 become simple. 



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