54 



ELEMENTARY TISSUES. 



substance. The small vascular 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 



FIG. 23. 



Enamel fibres (from Kolliker) ^f fi . A, fragments and single fibres of the enamel, 

 isolated by the action of hydrochloric acid. B, surface of a small fragment of enamel, 

 showing the hexagonal ends of the fibres. 



communication 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 inclosing it, and causing its ab- 

 sorption, 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 per- 

 manent teeth push their way from beneath, by gradual increase 

 and development, so as tb succeed them. 



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

 cisors, 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 num- 

 ber 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 



