344 MICROSCOPIC ANATOMY OF THE TEETH 



far from the external epithelium of the enamel organ dis- 

 appearing, it persists with the two inner epithelial layers, so 

 that we cannot avoid the conclusion that the membrane 

 consists of the altered cornified cells of the whole enamel 

 organ with the exception of the stellate reticulum, and also 

 includes other altered and degenerated cells of the follicle. 



The clear layer was considered by Kolliker to represent 

 a continuous structureless layer formed by the enamel cells 

 after their work is completed. 



Another view of its nature may be that it is a persisting 

 ameloblastic membrane, which according to Leon Williams 

 and many others is definitely to be seen between the ends 

 of the ameloblasts and the forming enamel, and corresponds 

 to one position of the ' membrana preformativa ' of Huxley. 

 If the internal ameloblastic membrane had not previously 

 disappeared, it would certainly come to occupy the position 

 of the clear layer of Nasmyth's membrane over the com- 

 pleted enamel, favouring the conclusions of Huxley and 

 Leon Williams that enamel is calcified by the dialysis of 

 lime salts through a membrane. As this dialysing membrane 

 persists while the tooth remains within the follicle, the cells 

 of Nasmyth's membrane would probably still be active and 

 functional, as they are in contact with the blood-vessels of 

 the capsule, and they thus may contribute to the final 

 consolidation of the enamel, as the author has shown is in all 

 probability the case, in the teeth of Sargus. It has been sug- 

 gested that after the eruption of the tooth, when the cells are 

 undergoing keratinization, the whole membrane may serve 

 as a dialysing membrane separating the lime salts from 

 the saliva. Pickerill, who adopts this view, says : ' The 

 enamel, after the eruption of the tooth, is covered with 

 a dead membrane, Nasmyth's membrane ; it is immersed 

 in a fluid saliva, containing in solution lime salts and 

 organic material (mucin and albumin), and it has been 

 shown that fluids with solids in solution can pass into the 

 outer layers of enamel ' (in experiments conducted by 

 Pickerill on the permeability of enamel from without). 

 ' It would seem therefore in every way probable that 

 Nasmyth's membrane acts as an ordinary dialysing mem- 

 brane, through which crystalloids pass, but colloids do not. 



