32 , ZOOLOGY. 



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. 



Fig. 29. Head of the ant-eater. 

 Myrmecophaga Jubata. 



Fig. 28. Head of the Boar. 



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. 20). 



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. 



56. This operation is an important one, inasmuch as the 

 more the food is masticated the easier will be the digestion. 

 When such instruments are wanting in> animals whose food 

 still requires trituration, this is effected by other means. The 

 gizzard, for example, in many birds is sufficiently strong to 

 answer this purpose. 



Insalivation. 



57. Whilst the food is undergoing trituration in the 

 mouth, it imbibes saliva, which sometimes even dissolves it. 



58. The saliva is formed partly in little mucous cavities 

 hollowed out of the mucous membrane of the mouth, partly 



