THE TEETH. 93 



the hardest materials; wherefore it follows that the inner 

 portion of each tooth is soonest to wear away. From this 

 arrangement, and due to the operation of this cause, it follows 

 that the outer crust of each of a rat's front-teeth will extend 

 in length, and form a cutting edge. In order to make this 

 arrangement effective in rodent animals, the longitudinal 

 growth of these chisel-teeth is very rapid; so rapid that 

 if an opposite corresponding tooth be drawn, whereby no 

 bearing-point shall be left, the unopposed tooth will continue 

 growing circularly until, curling round, its farther develop- 

 ment is stopped by pressure of the animal's own skull. A 

 preparation illustrative of this may be seen in the Museum of 

 the Royal College of Surgeons. 



Teeth, regarded as to material, are composed of a hard 

 outer covering known as enamel, and an inner portion of soft 

 bone furnished with nervous branches; as in toothache we 

 discover. Chemically, tooth- enamel is remarkable in the cir- 

 cumstance of its holding a considerable portion of fluoride of 

 calcium, the material of fluor, or Derbyshire spar, in point of 

 fact. The full complement of human teeth is thirty-two, 

 four of which, however, coming late, are called wisdom-teeth. 

 Everybody who is of suitable age remembers that, when a 

 child, his first teeth fell out, these having been the first set, 

 or milk-teeth. 



The coming of the milk-teeth belongs to those oblivious 

 days of infancy and early childhood which, going, leave no 

 memory behind. That order, however, was the following: 

 the milk, or deciduous, teeth were twenty in number, and 

 they made their appearance thus : first came the four central 

 incisors, about the seventh month after birth, but occasionally 

 earlier or later, those of the lower jaw appearing first ; next 

 in order came the lateral incisors, the lower jaw again having 

 precedence. Those teeth usually appear between the seventh 

 and tenth month. Then there was a short period of rest, 

 after which the front molars came forth soon after the twelfth 



