MASTICATION. 49 



a material innutritious because, when given alone, it is in- 

 sufficient to nourish. 



Fourthly, The proximate principles vary in proportions in 

 different lands of food, and a combination of various forms of 

 the latter is essential in all animals. 



The facts brought out in the foregoing pages are essential 

 in order that we may understand the process of digestion in 

 the lower animals, and after describing the changes which the 

 different principles undergo in the alimentary canal, I shall 

 refer to foods and the methods of feeding. 



MASTICATIOX. In many animals food has to be divided 

 and triturated in the mouth between teeth, which are lodged 

 in bony jaws, and these moved by powerful muscles. 



There are two distinct jaws : the upper, immovable ; and 

 the lower, which, by a joint with the temporal bone, moves 

 away from or closes on to the upper jaw. The character of the 

 joint indicates the habits of the animal The rodent is en- 

 dowed with back and forward movement by the disposition of 

 the articular head of the lower jaw, and its corresponding con- 

 cavity. In the carnivora a simple opening and closing or 

 champing is ensured by the arrangement of the bones, and in 

 herbivora a lateral and partially rotatory movement is possible. 



The teeth, lodged in the jaws for mastication, and acting 

 also as offensive and defensive organs, are either simple or 

 compound. Compound teeth are only observed in herbivorous 

 animals, presenting a surface composed of materials varying 

 in density and hardness, so as to ensure a constantly rough 

 surface for the purposes of grinding (see Fig. 25.) A good 

 millstone is composed of materials which wear with a different 

 degree of rapidity, and thus the surface rubs down most 

 effectually the materials over which it passes. The simple 

 tooth, as shown at Fig. 20, is all covered by solid enamel or 

 ivory, of a distinct white colour, and harder than any othei 



