NATURE OF THE CUTICULA DENTIS. 325 



external epithelium of tlie enamel organ, and which has 

 recently been spoken of by Professor Rolleston/ as being 

 represented by the sharp line which limits the enamel off 

 from the cement in sach teeth as the incisors of the wombat. 



And if any further proof were necessary to disprove this 

 latter hypothesis, it is to be found in the fact that a precisely 

 similar membrane may be raised from the unworn teeth of 

 many fish which have no enamel. 



Nasmyth's membrane, then, is continuous, not with the 

 whole thickness of the cementum, but with its outermost layer; 

 in other words, Avith the layer most recently formed. The 

 rapidity with which it disappears under the influence of 

 mastication, as well as the fact that it is rather deeply tinged 

 by nitric acid, point to its being not very fully, if at all, 

 calcified, an inference which is borne out by its being con- 

 tinuous with the outermost portion of cementum. Here the 

 researches of Mr. Rainie,^ which have been supplemented 

 by those of Professor Harting,^ throw considerable light on 

 the matter. By the latter observer it has been shown that 

 albumen, in the presence of certain lime salts, undergoes a 

 peculiar modification (to which he gives the name of " calco- 

 globulin"), conferring on it such a great power of resisting 

 acids that he states that it resembles chitin. It is noteworthy 

 that the linings of Haversian canals, encapsuled bone-lacunae, 

 the sheaths of the dentinal canals, and the youngest layer of 

 enamel, are all structures which have in common a very 

 remarkable power of resisting the action of acids, so that they 

 seem almost indestructible ; and they have this also in common, 

 they they lie between perfected calcified structures and the 

 soft tissues from which these are being formed. How far in 

 their chemical nature they may correspond with Professor 

 Harting's calco-globulin must for the present remain an 

 unsettled question ; this much, however, seems tolerably 

 certain, namely, that on the border of perfected calcified 

 structures there very generally exists a thin stratum of 

 tissue which presents marked chemical diff'erences from that 

 which lies on either side of it, one of the manifestations of 

 such difierence being that great indestructibility which 

 enables us to isolate it by the use of acids. 



Nasmyth's membrane appears to belong to this class of 

 structures ; it is not exactly perfected cementum, but it is 

 continuous with the outermost, and as yet unfinished, layers of 

 cementum ; still, it is homologous with the coronal cementum 



^ 'Translations of Odoutological Society,' 1271, vol. iii, p. 245. 



^ ' British and Foreign Medico-chirurgical Review,' No. xl, October, 1857. 



* ' Gluarterly Journal of Microscopical Science/ April, 1872. 



