ERUPTION OF THE TEETH. 191 



Various very strong objections have been brought forward, 

 clearly proving that this cause is quite inadequate to explain 

 all that may be observed. In the first place, teeth with very 

 stunted roots which may be practically said to have no 

 root are often erupted. Again, a tooth may have the 

 whole length of its roots completed, and yet remain buried 

 in the jaw through half a person's life, and then, late in life, 

 be erupted. Moreover, when a healthy normal tooth is being 

 erupted, the distance travelled by its crown materially ex- 

 ceeds the amount of addition to the length of its roots which 

 has gone on during the same time. 



To turn to comparative anatomy, the tooth of a crocodile 

 moves upwards, tooth pulp and all, obviously impelled by 

 something different from mere elongation; and my own 

 researches upon the development and succession of rep- 

 tilian teeth clearly show that a force quite independent of 

 increase in their length shifts the position of, and " erupts" 

 successive teeth. But what the exact nature of the impulse 

 may be, is an unsolved riddle : the explanations which I have 

 read, being, to my mind, less satisfying than the admission 

 that we do not know. 



V-^Towards the eighth month of childhood the bony crypts 

 / which contain the temporary teeth in the front of the mouth 

 begin to be renewed. The process of absorption goes on 

 with greater activity over the fronts of the crowns than over 

 their apices, so that almost the whole outer wall of the 

 alveoli is removed. At the back of the mouth the crypts 

 still retain their inverted edges ; indeed, development of the 

 crypts is still going on in this part of the mouth. 



When a tooth is about to be cut, very active absorption of 

 its bony surroundings goes on, particularly on the anterior 

 surface, the bone behind it being still required as forming 

 part of the crypt of the developing successional tooth. But 

 no sooner has the crown passed up through the very wide 

 and free orifice so formed, than absorption gives place to 



