MASTICATION. 121 



MASTICATION. In the first division of the alimentary canal, viz., 

 the mouth, the food undergoes simultaneously two different opera- 

 tions, viz., mastication and insalivation. Mastication consists in 

 the cutting and trituration of the food by the teeth, by the action 

 of which it is reduced to a state of minute subdivision. This pro- 

 cess is entirely a mechanical one. It 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 it be brought in 

 contact with the digestive fluids in a solid mass. This is always 

 the case when a solid body is subjected to the chemical 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 or decomposed 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 swallowed 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 (Fig. 19), 

 and arranged in a double or triple row 



**' TfifT 1 Q 



about the edges of the jaws, and sometimes 

 covering 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 CarmVOrOUS quadrupeds, as those Of SKTLL OP RATTLESNAKE. 

 . , , , . , , - . ., (After Achille-Richard.) 



the dog and cat kind, and other similar 



families, there are three different kinds of teeth adapted to different 

 mechanical purposes. (Fig. 20.) First, the incisors, twelve in num- 

 ber, situated at the anterior part of the jaw, six in the superior, 

 and six in the inferior maxilla, of flattened 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 molars, eight or more in number on each side, are 

 larger and broader than the incisors, and provided with serrated 



