174 THE VITAL FUNCTIONS. 



applied to each other are mutually adapted in 

 their curvatures ; a concave surface being every 

 where applied to one which is convex : on the 

 left side, the concavity is above ; but on the 

 right side, it is below. The horny covering is 

 much stronger, and more rough than in the 

 turkey ; so that the food is ground by a sliding, 

 instead of a rotatory motion of the parts opposed ; 

 and they do not require the aid of any inter- 

 vening hard substances of a large size. This 

 motion bears a great resemblance to that of 

 the grinding teeth of ruminating animals, in 

 which the teeth of the under jaw slide upwards, 

 within those of the upper, pressing the food be- 

 tween them, and fitting it, by this peculiar kind 

 of trituration, for being digested.* 



§ 6. Deglutition. 



The great object of the apparatus which is to 

 prepare the food for digestion, is to reduce it 

 into a soft pulpy state, so as to facilitate the 

 chemical action of the stomach upon it: for 

 this purpose, solid food must not only be sub- 

 jected to mechanical trituration, but it must 

 also be mixed with a certain proportion of fluid. 

 Hence all animals that masticate their food are 



* Home, Phil. Trans, for 1810, p. 188. 



