NASMYTH'S MEMBRANE 345 



Therefore, other things being equal, and so long as the 

 lime salts, especially the calcium phosphate, remain in 

 solution, they must tend to pass through the membrane 

 and penetrate the enamel, and the mucin and albumin are 

 kept back. No doubt the process is very slow and gradual, 

 depending largely on the relative osmotic pressures on either 

 side of the membrane, but it must undoubtedly take 

 place ' (7). 



These considerations are, however, more or less of a 

 speculative nature, and it is difficult to arrive at any definite 

 conclusions with regard to the functions of Nasmyth's 

 membrane ; still it would appear that, given a membrane 

 separating a colloid from a crystalloid, diffusion must, by 

 the laws of osmosis, take place through it. It has been 

 considered by some that Nasmyth's membrane acts as 

 a protective covering to the enamel, preserving it from the 

 action of acids, but it is difficult to conclude how far this 

 can be the case, for in healthy conditions any acid normally 

 present in the mouth would not be likely to have any in- 

 jurious effect upon the enamel, and we cannot consider that 

 Nasmyth's membrane has been evolved for the protection 

 of the teeth from a pathological process such as caries. 



So long as it is a continuous membrane it would no doubt 

 serve the latter purpose, but when detached in places, would 

 in all probability rather favour the process than otherwise, 

 as bacteria would proliferate beneath the separated mem- 

 brane, which would retain their acid products in contact 

 with the enamel surface. 



In view of the direct evidence of the cellular nature of 

 Nasmyth's membrane it is scarcely necessary to discuss at 

 any length the earlier views of its origin. Although not the 

 usual function of the follicle in its coronal portion to lay 

 down cement in teeth the crowns of which are not normally 

 covered with this tissue, it seems theoretically conceivable 

 that a kind of attempt to do so might occasionally result in 

 the deposition of a thin layer outside Nasmyth's membrane. 



It seems, however, highly improbable, and we cannot but 

 think that, as has been suggested, the so-called encapsuled 

 lacunae may be isolated cells of the enamel organ, especially 

 as a thin layer of cement does not show lacunae in its 



