XI ENTERIC CANAL 515 



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 constriction 

 is seen on the outside marking the attachment, on the 

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

 tine 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 mesen- 

 tery (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 differ- 

 entiated 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 

 (compare p. 207). 



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 honey- 

 combed 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 em- 

 bedded. 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, 

 is lined with similar lymphoid tissue. The mucous 



