THE DENTAL TISSUES. 103 



colour. The occurrence of lacunee in these situations is far 

 from rare : my father's collection contains more than a dozen 

 good examples of them in these positions. 



Nasmyth's membrane, thin though it is over the exterior 

 of the enamel, is thickened when it covers over a pit or 

 fissure, and when isolated by an acid is seen to have entirely 

 filled up such spots. (Fig. 55). 



In these places, then, where the encapsuled lacunae are to 

 be found, Nasmyth's membrane also exists, a fact which 

 alone would lend some probability to the view that it is 

 eementum. 



The general absence of lacunae in Nasmyth's membrane 

 is due to the fact that it is not thick enough to contain them ; 

 just as the thinnest layers of unquestionable cementum also 

 are without lacunae. 



In sections of an unworn bicuspid which was treated with 

 acid subsequently to its having been ground thin and placed 

 upon the slide, I have several times been fortunate enough 

 to get a view of the membrane in situ ; it then appears to 

 be continuous with an exterior layer of cementum, which 

 becomes a little discoloured by the acid employed to detach 

 Nasmyth's membrane from the enamel. I am therefore 

 inclined to regard it as young and incomplete cementum, 

 and to consider it as representing (upon the human tooth) 

 the thick cementum which covers the crowns of the teeth of 

 Herbivbra ; and I am very glad to learn from my friend 

 Dr. Magitot, who has made many as yet unpublished re- 

 searches upon this subject, that he entirely concurs in this 

 view, which has also the support of Professor Wedl. 



The evidence offered that Nasmyth's membrane is cementum, 

 although strong, does not amount to absolute proof ; it is therefore 

 desirable to briefly recapitulate the other explanations of its nature 

 which have been offered. 



Nasmyth, who first called attention to its existence, regarded it 

 as " persistent dental capsule ; " a view of its nature not -very ma- 

 terially differing from that advocated in these pages. 



