THE TEETH. 327 



jecte from the gum into the cavity of the mouth, and the lat- 

 ter that which is buried in the socket. } The incisors, eye 

 teeth and bicuspids have one root each, the upper grinders 

 three and the lower grinders two. The roots of the wisdom 

 teeth are commonly compressed into one mass having little 

 or no divergency, and in rare instances the bicuspids have 

 two distinct fangs, while the upper molars have been known 

 to have four or five distinct roots. 



5. {fevery human tooth is composed of two distinctly or- 

 ganised substances, namely the enamel covering the crown, 

 and the osseous or bony portion which constitutes the re- 

 remainder of the tooth.,) The osseous part is scarcely distin- 

 guishable from other bony structures excepting from its 

 greater density. The enamel on the contrary, is remarka- 

 ble for its extreme hardness resulting from its chrystaline 

 structure. It consists of needle-formed crystals, one ex- 

 tremity of which rests upon the bone beneath, while the 

 otheiys presented to the food. 



6.( The teeth are found by chemical analysis to consist of 

 nearly the same elementary substances as the other bones ; 

 namely, phosphorus, lime, magnesia, soda, carbon, oxygen, 

 chlorine, gelatine and watery (As the teeth are a part of the 

 living system, they are supplied with nerves, blood vessels 

 and absorbents, not only by a central cavity extending from 

 the point of each root to the middle of the crown, but by a 

 membranous sheath enveloping the root called the periosteum, 

 as is true of the other bones.) 



7. frhe teeth are firmly attached to the jaw or maxillary 

 bone, by means of sockets called alveoli^ fin case of extrac- 

 tion the alveoli yield to moderate pressure and suffer the 

 teeth to escape without any injury to the adjacent parts)^ 

 and as soon as the teeth are removed the investigating bone 

 is sufficient to produce immediate absorbtion. 



8. iThe first set of teeth begin to make their appearance 

 through the gums, between the sixth and eighth month} and 

 before the infant arrives at the thirtieth month all the teeth 



