74 THE PERMANENT DENTITION. 



met with in other animals. They are thrust up from 

 the alveoli during the entire life of the animal to re- 

 place the surface worn by friction." 



The activity of the growth of the grinders is re- 

 markable about the seventh year, for at this time 

 their roots begin to develop ; growth is thus going on 

 at both ends at the same time. A 

 third movement is now at least 

 apparent, for the undivided base 

 in the socket appears to be slowly 

 pushed out, which may partly ac- 

 count for the shrinkage of the 

 gums. The tenacity of the adhe- 

 sion of the periosteum would not wholly prevent this 

 movement, for it acts as a cushion, its elasticity pre- 

 venting concussions. The undivided base resembles 

 a post set in the ground, except that the implanted 

 part is smaller than the crown. 



Up to about the sixteenth year, the growth of the 

 teeth results chiefly from vitality transmitted through 

 the medium of the pulp. After the pulp has become 

 converted into dentine, however, the tooth " draws its 

 nourishment from the blood-vessels of the socket."* 



Surgeon Louis Brandt (" The Age of Horses," In- 

 dianola, Texas, 1860) says of the incisors : 



" The length of the teeth is constantly decreasing, 

 and often quite regularly, so that in extreme old age 

 they will sometimes not exceed half an inch in length, 

 while at their prime they were 2 to 3 inches long 

 Their breadth decreases nearly in proportion to the 

 decrease in length." 



* See pages 169-70. 



