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 rela- 

 tion 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 also of the general principle 

 that vegetable aliment requires, for this purpose, 

 longer processes, and a more complicated appa- 

 ratus than that which has been already ani- 

 malized. In the herbivorous tribes, we find the 

 oesophagus either extremely dilatable, so as to 

 serve as a crop, or receptacle for containing the 

 food previously to its digestion, or having a dis- 

 tinct pouch appended to it for the same object ; 

 to this there generally succeeds a gizzard, or ap- 

 paratus for trituration, furnished, not merely with 

 a hard cuticle, as in birds, but also with numerous 

 rows of teeth, of various forms, answering most 



