324 CHARLES S. TOMES. 



acid (1 part to 10 parts water) ; as the enamel dissolves away 

 Nasmyth's membrane will be seen to detach itself from the 

 surface ; so soon as this has taken place the acid is to be 

 removed with blotting-paper, and the section very carefully 

 washed so as not to disturb the relative position of the mem- 

 brane and the tooth, and, after drying, mounted in Dammar 

 varnish. If the manipulations have been carefully conducted 

 a preparation like that represented in fig. 6 Avill be the result. 



In this specimen the enamel (e) is jjartly dissolved away, 

 whilst Nasmyth's membrane (a) is seen but little moved from 

 its original position ; at the iipper and right-hand part of the 

 figure it is seen to be continuous and identical with the 

 dark contents of a deep fissure in the enamel (d) ; previous to 

 the application of the acid no histological characters could 

 be made out in the mass, owing lo its great opacity, though 

 the shape and size of the fissure would lead to the expecta- 

 tion of finding lacunse in it. On tracing the membrane down 

 the surface of the tooth it is seen to be torn in one place, 

 though none of it has been lost ; the lower fragment («') is 

 seen to pass Avithout break of continuity far beloAv the neck 

 of the tooth, where the oi'dinary cement (c) exists. 



But it will be observed in the figure that, although the 

 cement is at this part of the tooth very thin, it is not the 

 whole thickness which is continuous with Nasmyth's mem- 

 brane ; and this is more noticeable in the specimen itself, 

 in Avhich the membrane, as well as the outermost portion of 

 the cement, has a yellowish tinge. 



If a section be made of the calcified portion of the tooth of 

 a calf at that time when the enamel has just attained its full 

 thickness and the deposition of cement upon it is about to 

 commence, and the section be treated with acid in the manner 

 described, a membrane precisely similar to Nasmyth's mem- 

 brane in the human tooth peels off; but if a somewhat older 

 tooth be selected, in which the coronal cementum has attained 

 an appreciable thickness, no membrane is made to peel oiF 

 from its outer surface. The explanation of this fact is 

 obvious : when the structure in question is attached only to 

 enamel it is set free by the action of an acid, seeing that the 

 enamel is absolutely dissolved away, but when it is attached 

 to cementum which is only decalcified, but not dissolved 

 away, it is not set free. 



The yellowish colour which characterises Nasmyth's 

 membrane after treatment with acids (particularly with 

 nitric acid) is, however, to be seen on the outside of the 

 cementum, and not betiueen the enamel and the cementum, 

 which last position it would hold if it were formed from the 



