180 THE VITAL FUNCTIONS. 



Among the invertebrated classes we occasionally 

 meet with instances of structures exceedingly ana- 

 logous to a gizzard, and probably performing the 

 same functions. Such is the organ found in the 

 Sepin : the Earth-worm has both a crop and a 

 gizzard ; and Insects offer numerous instances, 

 presently to be noticed, of great complexity in the 

 structure of the stomach, which is often provided, 

 not only with a mechanism analogous to a gizzard, 

 but also with rows of gastric teeth. 



Chapter VIII. 



CHYLIFICATION. 



The formation of Chyle, or the fluid which is the 

 immediate and exclusive source of nutriment to 

 the system, takes place in the intestinal tube, into 

 which the chyme prepared by the stomach is re- 

 ceived, and where further chemical changes are 

 effected in its composition. The mode in which 

 the conversion of chyme into chyle is accom- 

 plished, and indeed the exact nature of the changes 

 themselves, being, as yet, very imperfectly known, 

 it is consequently impossible to trace distinctly the 

 correspondence which, in all cases, undoubtedly 

 exists between the objects to be answered and the 

 means employed for their attainment. No doubt 

 can be entertained of the importance of the func- 

 tions which are performed by structures so large 

 and so complicated, as those composing the alimen- 

 tary canal and its various appendages. We plainly 



