xi ENTERIC CANAL 515 



or the frog, and only reaching such a relatively large size 

 as in the rabbit in certain other herbivorous Mammals in 

 which the stomach has a simple form : in those which 

 possess a complicated stomach (viz., Ruminants) the caecum 

 is comparatively small. It is continuous with the proximal 

 end of the colon, which contains an intra-colic valve and 

 into which the round sac at the distal end of the ileum 

 opens by a circular aperture provided with an ileo-colic 

 valve. From this point arises the thin-walled caecum, which 

 lies coiled on itself amongst the folds of the rest of the 

 intestine : it is about an inch in diameter, and a spiral con- 

 striction is seen on the outside marking the attachment, on 

 the inside, of a spiral valve like that of the dogfish's in- 

 testine but narrower which makes about twenty-four turns 

 and ends at the base of a blind, finger-shaped process, the 

 vermiform appendix, which forms the apex of the caecum. 

 The whole canal is supported by a mesentery (p. 27), the 

 right and left halves of which are united, and which has a 

 very complicated arrangement in correspondence with the 

 numerous folds of the intestine. 



It will be noticed that the intestine is much more differentiated as 

 regards its subdivisions than in the Vertebrates previously examined, and 

 also that it is relatively much longer, being fifteen or sixteen times as 

 long as the body. 



On cutting open the small intestine, its mucous membrane is seen to 

 be raised into minute, finger-shaped elevations or villi, and here and 

 there certain patches present a honeycombed appearance : these portions 

 are known as Peyer's patches^ and, like the tonsils, thymus, and spleen, 

 consist of masses of lymphoid follicles composed of a connective-tissue 

 framework in which numerous leucocytes are imbedded. Other 

 so-called " lymphatic glands" or adenoids are present in the mesentery 

 and elsewhere. Peyer's patches also occur in the proximal end of the 

 colon, close to the ileo-colic aperture ; and the round sac with which the 

 colon communicates, as well as the vermiform appendix, are lined with 



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