i8 



ELEMENTS OF PHYSIOLOGY 



enamel, a substance harder, denser, and more shining than 

 the dentine. 



23. The enamel is worn off from the top of the crown 

 between the twentieth and thirtieth years. (If you examine 



a person's mouth in whose teeth the 

 yellowish dentine shows through the 

 white enamel, you will know that per- 

 son to be at least how old ?) Although 

 the dentine is exposed through the 

 wearing or breaking of the enamel, it 

 will last for years without decaying, but 

 it is more likely to 

 decay than the en- 

 amel. We should 

 never run the risk 

 of breaking the 

 enamel by sudden 

 slipping of the 

 teeth, such as often 

 occurs when eating 

 hard candies or 

 cracking nuts. A 

 tooth should never 

 be pulled if it can 



be saved by being filled ; for pulling 

 a tooth means not only the loss of one 

 tooth, but the end of usefulness of the 

 tooth opposite to it, and imperfect chew- 

 in g in that part of the mouth. Losing FIG. 22. Teeth of a Cow 



fed on Slops. 



a tooth is equivalent to losing part of Showing ^ Q{ eating 



the life. fd that requires the use 



24. Decay of the teeth is caused ( i ) by 



using only soft food (Figs. 21 and 22) which requires no 

 pressure of the teeth while chewing it ; (2) by particles of 

 food lodging between the teeth and becoming quickly de- 



FlG. 2i. Teeth of a 

 Healthy Cow. 



