88 



ELEMENTARY PHYSIOLOGY 



the age of about seven years these milk teeth begin to be cast off, 

 their places being taken by the permanent teeth which grow be- 

 neath them ; and, at about the age of twelve, all have in this way 

 given place to the permanent set 



The permanent teeth of the adult number thirty-two. They 

 consist, in each jaw, of four incisors, two canines, four bicuspid, and 



J7I.1 



rns. 



Fang 



Fang 



Fig. 85. Showing the 

 four kinds of Human 

 Teeth. 



i, incisor tooth (exterior view) ; 

 2, canine tooth (exterior 

 view) ; 3, bicuspid tooth (side 

 view showing the two cusps) ; 

 4, molar tooth (exterior view). 



Fig. 86. Section of the Jaws of a Child of 6 l , 

 Years, showing the Milk or Deciduous Teeth, 

 also the Permanent Teeth in Process of 

 Formation. 



di, the milk incisors ; dc, the milk canines ; d$, and d$, the 

 milk molars ; z"i and Z2, the permanent incisors ; c, the 

 permanent canines ; /3 and/4, the permanent bicuspids ; 

 mi, the first permanent molars, which have already made 

 their appearance ; >2 and m$, the second and third 

 permanent molars. 



six molars. The last of the molars are called the wisdom teeth; they 

 do not appear till between the seventeenth and twenty-fifth years. 

 The mass of a tooth consists chiefly of a hard substance called 

 dentine or ivory (Lat. dens, a tooth). It resembles in composition 

 the compact tissue of bone, but contains a much larger proportion 

 of mineral matter (seventy-two per cent), and is consequently 

 harder. The mineral matter, like that in bone, consists chiefly of 

 calcium phosphate and calcium carbonate ; and the animal matter 

 may also be made to yield gelatine by boiling. In structure dentine 

 differs from bone, as it contains no Haversian canals, lacunae, &C., 



