NASMYTH'S MEMBRANE 347 



that has been brought forward is sufficient in itself to 

 account for the phenomenon. 



In a recent paper by Warwick James and A. T. Pitts, 

 the authors consider that there are two chief factors con- 

 cerned in eruption (16). 



(1) A process of advancement of the tooth in the tissues. 



(2) A process of denudation by absorption of the tissues 

 overlying and surrounding the tooth. 



In the process of advancement they consider that the 

 point of eruption is determined by the presence of the 

 epithelial columns connecting the oral epithelium with that 

 lining the tooth follicle. The advancement of the tooth is 

 partly due to unequal rates of growth between the various 

 tissues surrounding the tooth, and they consider that the 

 elongation of the root plays some part in eruption. 



As pointed out by Tomes (17), the elongation of the root 

 is alone quite inadequate to produce the effect. Teeth with 

 stunted roots are frequently erupted, and a tooth with 

 fully completed root may remain within the jaw and erupt 

 late in life. 



He compares eruption in man with that in reptiles, 

 showing that ' a force quite independent of increase in 

 length shifts the position of and " erupts " successive 

 teeth '. 



Messrs. James and Pitts compare the eruptive process to 

 ' the opening of a book, the hinged portion being advanced, 

 pari passu, with the separation of the pages of the volume, 

 until it comes to occupy the same level as the free edges '. 

 This implies the movement of the tooth, the cause of which 

 still has to be accounted for ; for while the degeneration 

 and absorption of the cells of the follicle and capsule, and 

 especially the opening out of the epithelial coils (figs. 203 and 

 204) described on p. 312, would afford a path for the tooth 

 to the surface, it must still be moved forward in this path 

 to its final position in the jaw. 



Blood pressure was considered by Constant (13) to be the 

 cause, of the movement of the tooth, the vascular tissues 

 beneath the tooth serving as the propulsive force. It seems 

 very probable that this is one of the factors in eruption, 

 but not sufficient in itself to account for the whole process. 



