DIGESTIVE ORGANS AND THEIR ANATOMY. 



271 



in it, much decayed food material, in which case the tartar 

 itself with these decaying impurities in it becomes a source 

 of infection and needs immediate removal. 



The hygiene of the teeth forms such an integral part of 

 a person's general sense of order and cleanliness that it 

 would be entirely out of place to comment further on it in 

 this connection. 



The Development of the Teeth. 



As pointed out previously, the* teeth when once fully 

 formed are not able to grow or repair themselves in any ma- 

 terial way. This is absolutely true of the 

 enamel. After that has been formed in 

 the first appearance of the tooth, it is 

 never possible to be replaced later. How- 

 ever, with the dentine a slight repairing 

 action may be noticed. If, for instance, 

 on the crown or neck of the tooth where 

 the enamel has been removed, the dentine 

 has been worn away and a cavity so re- 

 sulted, there is not infrequently deposited 

 in the pulp cavity on the dentine in a 

 position corresponding exactly with the 

 corroded place on the outside a new bit 

 of bone-like dentine which serves to coun- 

 teract, to some extent at least, the corro- 

 sive action on the outside. Even the 

 dentinal tubules which extend into the 

 dentine so softened or corroded become 



Fig. 103. LONGITUDINAL o 11J . 1 



SECTION OF AN INCISOR filled with calcareous depositions and the 



rT G D T T ' R ; tooth so becomes firmer and consequently 

 AND /, POINTS CORRE- more resistant to decay. Reference to 



, . ... .-, , . 



the accompanying diagram will explain 

 t hi s> The cement of the teeth is within 

 certain limits easily replaced. It is prac- 

 tically nothing but bone and is secreted by a kind of peri- 

 osteum which covers the jaw-bone and which dips into the 



SPONDING TO PLACES OF 



DENUDED DENTINE ON 

 THE EXTERIOR. (After 



Salter.) 



