150 THE VITAL FUNCTIONS. 



opening laterally from the cavity of the intestine, and having 

 no otlicr outlet. Structures of this description have already 

 hccn noticed in the infusoria,! and they are met with, indeed, 

 in animals of every class, occurring in various parts of the 

 alimentary tube, sometimes even as high as the pyloric por- 

 tion of the stomach, and frequently at the commencement 

 of the small intestine. Their most usual situation, however, 

 is lower down, and especially at the part where the tube, af- 

 ter having remained narrow in the first half of its course, is 

 dilated into a wider cavity, which is distinguished from the 

 former by the appellation of the great intestine, and which 

 is frequently more capacious than the stomach itself. It is 

 exceedingly probable that these two portions of the canal 

 perform different functions in reference to the assimilation 

 of the food: but hitherto no clew has been discovered to guide 

 us through the intricacies of this difficult part of physiology; 

 and we can discern little more than the existence 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 ca:ca, in animals that feed on ve- 

 getable substances, than in carnivorous animals of the same 



class. 



The class of insects supplies numberless exemplifications 

 of tlie accurate adaptation of the structure of the organs of as- 

 similation to the nature of the food which is to be convert- 

 ed into nutriment, and of the general principle that vegeta- 

 ble aliment requires longer processes and a more compli- 

 cated apparatus for this purpose, than that which has been al- 

 ready animalized. In the herbivorous tribes, we find the oeso- 

 phagus cither extremely dilatable, so as to serve as a crop, or 

 receptacle for containing the food previous 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 



• Page 73, of this volume. 



