40 



MASTIFf CATlGft . 



grinding teeth, and when all are cut amount to twenty, 

 making the whole number of the teeth thirty two. The 

 last tooth in each jaw is not cut till about the twentieth 

 year, and is called the wisdom tooth. The teeth are now 

 firmly fixed in the jaw in cells or sockets formed by thin 

 plates of bone rising up from the jaw, as you may see in 

 the figure below. The only use of these sockets is t 



contain the teeth, and when they are no longer wanted in 

 consequence of the loss of the teeth, they are absorbed. 



Emily. Is not this the cause of that approximation of 

 the chin and nose so striking in old people who have 

 Jost their teeth ? It would seem that the sockets of both 

 jaws being taken away the distance between these two 

 parts of the face must be considerably diminished. 



Dr. B. Yes ; your explanation is correct. We are 

 now prepared to consider the action of the mouth on the 

 food, or what physiologists call the process of masti- 

 cation. The food taken into the mouth is in the first 

 place conveyed back and forlh between the teeth by the 

 tongue, the incisors cutting it in pieces, and the molares 

 grinding or breaking it down. This action of the rnol- 

 ares is particularly necessary when the food consists of 

 grains or hard fruits, and you see row how much this 

 grinding process is facilitated by the lateral motion of the 

 jaws. 



Emily. And I suppose the reason why the cat does 

 /not possess this lateral motion of the jaws, is that its 



