382 THE TISSUES. 



II. PERMANENT TEETH. 



Central incisors .... 8 years. 



Lateral " . . . . 9 " 



First bicuspid . . . . 10 " 



Second " .... 11 " 



Canines 12 to 12 \ years. 



Second molars .... 12 J to 14 " 



Third " 17 to 19 " 



Growth of the Teeth. 



The enamel does not increase in amount after the eruption of the 

 teeth. It is, however, susceptible of some molecular changes, as its ' 

 diseases indicate especially caries. The latter is true of the den- 

 tine also, and the cementum; both of which, moreover, become 

 thicker after the eruption of the teeth. Indeed, in old persons the 

 pulp has sometimes entirely disappeared, and its cavity been filled 

 with an imperfect dentine; and the cementum amounts to an exos- 

 tosis (p. 374). The fissures between the enamel-prisms, the dentinal 

 tubuli, and the lacunae and pores of the cementum all during life 

 contain a nutritive fluid, and permit of nutritive changes. Since, 

 however, perfect dentine is not colored by madder when an animal 

 is fejj with it, it is probable that these changes are far less active 

 than in the bones. In case of caries threatening an exposure of the 

 pulp of the teeth, the dentine very often becomes thicker within, 

 and opposite the carious portion ; the new dentine being formed to 

 protect the pulp. 



Any portion of a tooth being once removed, is never reproduced ; 

 nor is the loss at all repaired. 



A third dentition sometimes occurs late in life, though the teeth 

 are imperfectly developed and few in number. A tooth extracted 

 and at once replaced, may become firm again 'at the end of some 

 months (fifteen in one case). 



Pathological States of flue Teeth. 



1. Hypertrophy of the cement (exostosis), deposits of dentine 

 projecting into the pulp cavity, and ossification of the pulp itself, 

 are very common results of chronic inflammation of the perios- 

 teum and the pulp. In the first, the pores become dilated, so as to 

 form Haversian canals. (Wedl.) 



2. A partial disappearance of the fang is quite common. The 

 whole fang sometimes becomes transparent like horn. 



