136 DIGESTION. 



time that they undergo the process of liquefaction. The active agent 

 in producing this change is in every instance an albuminoid or nitro- 

 genous matter, which forms the most important ingredient in the 

 digestive fluid; and which, by coming in contact with the food, exerts 

 upon it a peculiar action, transforming its ingredients into new sub- 

 stances. It is these newly-formed materials which are finally absorbed 

 by the vessels and mingled with the general current of the circulation. 

 In the human species the first process to which the food is subjected is 

 that of mastication, while it is at the same time mingled with the saliva 

 in the cavity of the mouth. 



Mastication. 



Mastication consists in the cutting and trituration of the food by the 

 teeth, by which it is reduced to a state of minute subdivision. The 

 process is entirely a mechanical one, and is necessary in order to pre- 

 pare the food for the subsequent action of the digestive fluids. As 

 this action is chemical in its nature, it will be exerted more promptly 

 and efficiently if the food be finely divided than if brought in con- 

 tact with the digestive fluids in a solid mass. This is necessarily 

 the case when a solid body is subjected to the action of a solvent 

 fluid ; since, by being broken up into minute particles, it offers a larger 

 surface to the contact of the fluid, and is more readily attacked and 

 dissolved by it. 



In the structure of the teeth, and their physiological action, there are 

 certain marked differences, corresponding with the habits of the animal 



and the kind of food upon which it subsists. 

 In fish and serpents, in which the food is swal- 

 lowed entire, and in which the process of 

 digestion, accordingly, is comparatively slow, 

 the teeth are simply organs of prehension. 

 They have generally the form of sharp, curved 

 spines, with their points set backward, and 



SKITLL OF RATTLESNAKE. , . , . , 



(Achiiie Richard.) arranged in a double or triple row about 



the edges of the jaws, and sometimes cover- 

 ing the mucous surfaces of the mouth, tongue, and palate. They serve 

 merely to retain the prey, and prevent its escape, after it has been 

 seized by the animal. In the carnivorous quadrupeds, there are three 

 different kinds of teeth, adapted to different purposes. (Fig. 30.) 

 First, the incisors, twelve in number, situated at the anterior part of 

 the jaw, six in the superior, and six in the inferior maxilla, of flat- 

 tened form, and placed with their thin edges running from side to side. 

 The incisors, as their name indicates, are adapted for dividing the food 

 by a cutting motion, like that of a pair of shears. Behind them come 

 the canine teeth, or tusks, one on each side of the upper and under jaw. 

 These are long, curved, conical, and pointed ; and are used as weapons 

 of offence, and for laying hold of and retaining the prey. Lastly, the 



