388 



RODENTIA. 



fications to be noticed, both in the construc- 

 tion of the caecum and of the commencement 

 of the colon, which generally presents the 

 same appearance as the caecum itself for a 

 short distance from its commencement. 



The greater or less development of the 

 caecum is in relation to the nature of the food 

 appropriate to each individual. In one genus 

 only, namely, the dormouse (Myoxus}, it is 

 altogether wanting; those Rodents that live 

 upon grass and herbs have it most remark- 

 ably developed ; and in the hare it has been 

 calculated that the capacity of the caecum is 

 ten times as great as that of the stomach 

 itself. In the granivorous genera its size is 

 likewise very considerable ; so that, in the 

 hamsters, lemmings, Guinea-pigs, and allied 

 genera it has been estimated to be four times 

 larger than the stomach. 



Another remarkable peculiarity may be 

 observed in the caecum of the Rodentia, 

 namely, that it frequently has its cavity divided 

 into regularly arranged cells disposed in 

 several rows, or else forming a single series. 

 In other cases the cavity of the caecum is 

 divided into compartments by a broad spiral 



Fig. 212. 



Ccecum of the Squirrel. 



a, termination of the small intestine ; b, d, the 

 caecum ; e, dilated commencement of the colon. 



membrane. In some species again, as in the 

 jerboa, &c., the interior of the caecum is a 

 simple cavity, without any division or internal 

 complication. All these diversities of struc- 

 ture seem to be in relation with the different 

 kinds of food devoured by these animals. 



The proportionate length of the small in- 

 testine as compared with that of the large, is 

 frequently the reverse of what holds good in 

 carnivorous quadrupeds ; but the diameter of 

 the latter, except in the immediate vicinity of 

 the caecum, is scarcely greater than that of 

 the small intestine (fig. 273. m, w). 



The intestinal villi have the shape of leaf- 

 lets of fringed laminae, or sometimes of very 

 fine filaments ; the entire inner surface of the 

 small intestine is villous, whilst that of the 

 large intestine is quite smooth. 



Fig. 273. 



Stomach and intestinal canal of the Rat (Mus 

 Rattus). 



f, oesophagus ; a, b, d, compartments of the 

 stomach ; e, pylorus ; g, h, i, small intestine ; k, I, 

 caecum ; p, commencement of the colon ; m, n, colon ; 

 o, anus. 



It is worthy of observation that in those 

 species that have the caecum most largely de- 

 veloped, that organ is furnished with very 

 remarkable glandular appendages ; this struc- 

 ture is met with in the genera Lcpus and 

 Lagomys. 



In order to illustrate the above general 

 description of the digestive organs of the 

 order of quadrupeds under consideration, we 

 shall select a few examples illustrative of the 

 principal varieties which it presents in dif- 

 ferent genera. 



In the squirrel (Sciurus) the small intestine 

 (j%. 272. a) is nearly of the same diameter 

 throughout ; the caecum (b, c, d) is of 



valve, as is the case in the hares ; or, as in derate dimensions, of a conical shape, and 

 the marmots, by circular folds of its lining destitute of any cells or partitions internally. 



