276 PALiEONTOLOG\ 



The size of the perforation in the tooth, and of the depression 



in the jaw, proves them to have been in great part caused by 

 the soft matrix, which must have produced its effect by exciting 

 absorbent action, and not by mere mechanical force. The 

 resistance of the wall of the pulp-cavity having been thus 

 overcome, the growing tooth and its matrix recede from the 

 temporary alveolar depression, and sink into the substance 

 of the pulp contained in the cavity of the fully-formed 

 tooth. 



As the new tooth grows, the pulp of the old one is re- 

 moved ; the old tooth itself is next attacked, and the crown, 

 being undermined by the absorption of the inner surface of its 

 base, may be broken off by a slight external force, when the 

 point of the new tooth is exposed. 



The new tooth disembarrasses itself of the cylindrical base 

 of its predecessor (tig. 81, a) with which it is sheathed, by 

 maintaining the excitement of the absorbent process so long 

 as the cement of the old fang retains any vital connection with 

 the periosteum of the socket ; but the frail remains of the old 

 cylinder, thus reduced, are sometimes lifted out of the socket 

 upon the crown of the new tooth (as in fig. 81, a), when they 

 are speedily removed by the action of the jaws. This is, how- 

 ever, the only part of the process which is immediately pro- 

 duced by violence ; an attentive observation of the more 

 important previous stages of growth, teaches that the pressure 

 of the growing tooth operates upon the one to be displaced 

 only through the medium of the vital absorbent action which 

 it has excited. 



No sooner has the young tooth (fig. 80, h) penetrated the 

 interior of the old one (fig. 80, a) than another germ c. begins 

 to be developed from the angle between the base of the young 

 tooth and the inner alveolar process : or in the same relative 

 position as that in which its predecessor began to rise, and 

 the processes of succession and displacement are carried mi 



