CHYLIFICATION. 207 



assimilation of the food : but hitherto no clue 

 has been discovered to guide us through the 

 intricacies of this difficult part of physiology ; 

 and we can discern little more than the ex- 

 istence, already mentioned, of a constant relation 

 between the nature of the aliment and the 

 structure of the intestines, which are longer, 

 more tortuous, and more complicated, and are 

 furnished with more extensive folds of the inner 

 membrane, and with larger and more numerous 

 caeca, in animals that feed on vegetable sub- 

 stances, than in carnivorous animals of the same 

 class. 



The class of insects supplies numberless 

 exemplifications of the accurate adaptation of 

 the structure of the organs of assimilation to the 

 nature of the food which is to be converted into 

 nutriment, and of the general principle that 

 vegetable aliment requires longer processes, and 

 a more complicated apparatus, for this purpose, 

 than that which has been already animalized. 

 In the herbivorous tribes, we find the oesophagus 

 either extremely dilatable, so as to serve as a 

 crop, or receptacle for containing the food pre- 

 vious to its digestion, or having a distinct pouch 

 appended to it for the same object : to this there 

 generally succeeds a gizzard, or apparatus for 

 trituration, furnished, not merely with a hard 

 cuticle, as in birds, but also with numerous rows 

 of teeth, of various forms, answering most effec- 



