RODENTIA. 



389 



The colon (<?) is for a short distance almost 

 of the same diameter as the caecum, but it 

 soon diminishes in size, and throughout the 

 rest of its extent is scarcely wider than the 

 small intestine. Internally, it presents no 

 septa or valvulae conniventes. The intestinal 

 papillae form small lamellae, the borders of 

 which are fringed with delicate filaments ; 

 these papillae extend throughout the whole 

 length of the small intestine, but towards its 

 termination becomes smaller and less per- 

 ceptible. 



In the rats, the alimentary canal would be 

 nearly of the same calibre throughout, were 

 it not for the interposition of the caecum 

 between the ileum and the colon. The 

 caecum in this family of Rodents rather re- 

 sembles a second stomach (fig. 273. k, /) than 

 a bowel ; it is capacious, short, and slightly 

 curved upon itself, but without any constric- 

 tions, tapering gradually towards its blind ex- 

 tremity. The walls of the intestinal canal 

 are throughout thin, delicate, and transparent ; 



Fig. 274. 



Cacum of the Water Vole (Avicola ampldbius). 



I, m, end of the small intestine; n,o,p, q, caecum ; 

 r, dilated commencement of the colon ; s, point at 

 which the colon becomes contracted. 



but slight traces of a spiral valve are visible 

 at the commencement of the colon. 



In the water-rat (Arvicula amphibius) the 

 small intestines are of equable diameter 

 throughout their whole extent, but their 

 calibre is small, as indeed is that of the large 

 intestine. The caecum is, however, of enor- 

 mous proportions (Jig. 274. n, o, p y q), and is 

 divided at intervals into pouches by deep 

 constrictions. The commencement of the 

 colon (r) is extremely voluminous, but it soon 

 diminishes in its diameter, and is twisted in a 

 remarkable manner, so as to form several 

 close spiral turns ; the walls of the small in- 

 testine (/, m) are very thin and transparent ; at 

 the commencement of the colon its lining 

 membrane is thrown into regular folds, which, 

 as they appear through the transparent coats 

 of the intestine, resemble a series of spiral 

 muscular fibres. 



In other species belonging to the genus 

 arvicola, the same disposition is observable. 



In the Cape moles (Bathiergus) the struc- 

 ture of the caecum varies. In the orycterus 

 of the Downs (Bathiergus maritimus) the 

 caecum is short, and has its walls sacculated 

 and puckered up, as it were, by tendinous 

 bands. The colon begins by a wide pouch, 

 and preserves through nearly its whole length 

 a considerable diameter and sacculated ap- 

 pearance, but on approaching the anus it be- 

 comes contracted and of equable diameter. 



In the white-spotted orycterus (Bathiergus 

 capetisis) the caecum is much longer in pro- 

 portion and of more equal calibre, although 

 still very wide, in proportion to the size of the 

 small intestine, and much sacculated; the com- 

 mencement of the colon is at first of the 

 same diameter as the caecum, but it soon be- 

 comes narrower and spirally convoluted, 

 much in the same way as in the water-rat. 



In the hare and in the rabbit the small 

 intestine is nearly of the same diameter 

 throughout its whole length ; the caecum is of 

 a very remarkable size, and forms an enormous 

 elongated conical sac, divided, at intervals, by 

 deep constrictions into numerous compart- 

 ments, as far as about the distance of two or 



Fig. 275. 



Caecum of the Hare. 



c, termination of the ileum ; a, d, a spirally convoluted caecum ; b, d, its terminal glandular portion ; f t 

 dilated pouch, close to the termination of the small intestine ; e, capacious commencement of the colon 

 which, at g, becomes considerably diminished in size. 



c c 3 



