MASTICATION. 239 



In the bird, which is not supplied with a masticatory apparatus as 

 ordinarily understood, — that is, in whom mastication does not occur in 

 the mouth, — we have a supplementary organ, the gizzard, which serves the 

 same purpose in comminuting the food. In these animals, therefore, the 

 operation of mastication is performed in the abdominal organs and is 

 involuntary. Voluntary mastication performed in the mouth only occurs 

 in mammals, and is seen in its typical form in the herbivora, in a less per- 

 fect degree in the carnivora, while the omnivora occupy a mean between 

 the two. 



Mastication is a complex act, and requires the action of active and 

 passive organs; that is, the muscles of mastication, the jaws and teeth, 

 while it is aided by the tongue, lips, and cheek. In all vertebrates the 

 jaws move vertically, the nature and degree of the movement varying 

 with the nature of the food. In the carnivora the lower jaw is alone 

 usually movable, and its extent of motion is very much greater than in 

 the herbivora. The lower jaw is moved by five muscles on each side, the 

 temporal, the masseters, the two pterygoids, and digastric muscles, while 

 in the solipedes the stylo-maxillary muscle constitutes an auxiliary 

 muscle of mastication. The action which a muscle exerts is dependent 

 not only on the bulk of the muscle, but on the angle of insertion of the 

 muscle in the bone. The more acute the angle, and the nearer the point 

 of insertion to the articulation, the more extensive will be the excursion 

 of the movable part in the contraction of the muscle, and the greater 

 will be its velocity of movement. The more perpendicular the insertion 

 of the muscle, and the greater the distance between the point of insertion 

 and the articulation, the greater will be the power developed in the con- 

 traction of the muscle. The latter is the arrangement which generally 

 characterizes the muscles of mastication ; they are inserted perpendicu- 

 larly in the lower jaw, and at a considerable distance from the maxillary 

 articulation. The arrangement of the parts and the motions in mastica- 

 tion differ in different animals. 



In the carnivora the articulation of the lower with the upper jaw is 

 by a transverse condyle fitting into a canal-like groove in the temporal 

 bone, the canine teeth and molars overlap, and, the lower jaw being nar- 

 rower than the upper, the only motion therefore possible is a simple up 

 and down movement (Figs. 86 and 87). 



In herbivora the articulation of the lower with the upper jaw is above 

 the level of the molar teeth, and permits of a forward, backward, and 

 lateral, as well as an up and down, motion. Three distinct types of her- 

 bivora, with reference to their mode of mastication, may.be recognized. 



First, the rodents, which have only two kinds of teeth, two highly 

 developed incisors in each jaw ; the canine teeth are absent, while the 

 molars, which are compound teeth, have a flat crown and transverse rows of 



