MASTICATION. 29 



Of all the teeth the molars are the most useful ; hence their 

 presence is much more frequent than the incisives or ca- 

 nines. These latter, for an obvious reason, are never wanting 

 in the carnivora ; but they are not unfrequently absent in the 

 herbivora. The canine in some animals grow to a large size, 

 and become instruments of attack and defence (Fig. 21). 



Fig. 22. Head of the Ant-eater.* 

 Fig. 21. Head of the Boar. 



54. At birth it is seldom that the human teeth have cut 

 the gums ; they appear usually from six months to a year 

 after birth. The teeth which first appear are called milk 

 teeth, or deciduous, as destined to be thrown off and to be re- 

 placed by others. They are twenty in number, namely, in 

 each jaw ten viz., four incisives, one canine, and two molar. 

 About seven years of age these begin to fall or to be 

 thrown off, and to be replaced by another series of teeth, 

 situated in capsules imbedded more deeply in the jaws ; their 

 roots therefore are longer, and their insertion firmer. 



The teeth of the second dentition are more numerous than 

 those of the first : they are thirty-two in number, sixteen in 

 each jaw ; namely, four incisives, two canine, four small or 

 false molar, having each two roots, three true molars situated 

 behind these, and having each three roots (Fig. 16). 



In old age these permanent teeth fall as did the deciduous, 

 but they are not replaced. 



55. Mechanism of Mastication. The teeth, the pas- 

 sive instruments of mastication, are put in action by the 

 muscles of mastication acting on the jaws. The upper jaw, 

 forming a fixed portion of the head in mammals, moves only 

 with the head ; but the lower, by means of its articulations, 

 acts readily and powerfully, and by means of many muscles, 

 and with the aid of the tongue and cheeks, forces the food in 

 such a way between the surface of the grinding teeth as to 

 expose it fully to their action. 



* Myrmecophaga Jubata. 



