DEVELOPMENT OF TEETH. 55 



papilla is gradually encroached upon and imprisoned by 

 the calcareous deposit, until only a small part of it is left 

 as the tooth-pulp, which remains shut up in the harder 

 substance, with only the before-mentioned small communi- 

 cation with the outside, through the end of the fang. In 

 this manner the first set of teeth, , or the milk-teeth, are 

 formed; and each tooth, by degrees developing, presses 

 at length on the wall of the sac enclosing it, and causing 

 its absorption, is cut, to use a familiar phrase. 



The temporary or milk-teeth, having only a very limited 

 term of existence, gradually decay and are shed, while 

 the permanent teeth push their way from beneath, by 

 gradual increase and development, so as to succeed them. 



The temporary teeth are ten in each jaw, namely, four 

 incisors, two canines, and four molars, and are replaced by ten 

 permanent teeth, each of which is developed from a small 

 sac set by, so to speak, from the sac of the temporary tooth 

 which precedes it, and called the cavity of reserve. The 

 number of the permanent teeth is, however, increased to 

 sixteen, by the development of three others on each side of 

 the jaw after much the same fashion as that by which the 

 milk teeth were themselves formed. The beginning of 

 the development of the permanent teeth of course takes 

 place long before the cutting of those which they are to 

 succeed ; one of the first acts of the newly-formed little 

 dental sac of a milk-tooth being to set aside a portion of 

 itself as the germ of its successor. 



The following formula shows, at a glance, the com- 

 parative arrangement and number of the temporary and 

 permanent teeth : 



MO. CA. IX. CA. MO. 



( Upper 21412 =10 



Temporary Teeth . < = 20 



( Lower 21412 = 10 



MO.BI. CA. IX. CA. F>I. MO. 



(Upper 3 2 i 4 i 2 3= 16 



Permanent Teeth. =32 



(Lower 32 14123= 16 



