CHAPTEE XII. 



OF DIGESTION. 



A. The object of digestion is the preparation of assimilable 

 substances. The processes are the mecbanical division followed 

 by a chemical transformation of the aliments. In final analysis 

 absorption is only exercised on liquefied and dissolved substances. 



True digestion, therefore, does not exist among the beings 

 rudimentary, not differentiated, that form the first stage 

 of the animal kingdom. In an inferior degree it exists in 

 animals with a simple digestive tube, having neither at the 

 orifice of the alimentary canal, nor at any point of its cavity, 

 organs suitable for tearing, lacerating, and crushing, and conse- 

 quently not being able to assimilate easily almost anything but 

 aliments liquefied apart from their organism. 



Among animals possessing buccal, pharyngian teeth, or appara- 

 tus which occupy the place thereof, there exists usually a special 

 glandular apparatus charged to render alimentary trituration more 

 easy by imbibing the aliments with a liquid called salivary. In a 

 more general manner we may say that some kind of saliva exists 

 often when there is an apparatus for the prehension of aliments. 

 Thus the fly emits on the particles it is about to draw in with 

 its proboscis a brownish liquid which dilutes them. Naturally 

 the saliva is the more abundantly secreted the harder is the 

 habitual aliment. Thus the saliva is null or scanty in aquatic 

 fiiiimals, whatever they may be (crustaceans, fishes, crocodiles, 



