322 CHARLES S. TOMES. 



In support of this idea tlie fact that, by staining with 

 nitrate of silver, outlines resembling those of epithelial cells 

 may be brought out, is adduced. 



I am, however, unable to reconcile the appearances 

 observed with these hypotheses as to its origin; and the 

 object of the present paper is to endeavour to prove the 

 substantial correctness of a widely different view, not, indeed, 

 new, since it was advocated by Professor Owen^ and by my 

 father^ many years ago, and has been accepted by Professor 

 Wedl,^ but one Avhich has apparently been to some extent 

 over-looked, or, at all events, has not gained general credence. 

 The preparations at ray disposal appear to me to prove con- 

 clusively that the membrane is no way referable to the 

 enamel, but that it is nothing else than coronal cement ; that 

 it is, in fact, the homologue of the thick coronal cement of 

 herbivora. 



In the first place, it may be mentioned that a comparison 

 of the tooth-sacs of man with those of the herbivora reveals 

 no anatomical reason why there should not be coronal cement 

 on human teeth, as the histological elements present in the 

 one are also to be distinguished in the other. 



Indeed, it now and then happens that a deposition of well- 

 formed cementum actually does occur on the crown of a 

 human tooth. In the bicuspid tooth, a section from which 

 is represented in fig. 1, the cement of the fang ends at the 

 normal position around the neck of the tooth, where the 

 enamel commences. On one side of the crown there is, how- 

 ever, a roundish patch from which the enamel is entirely 

 absent, its place being taken by a thick deposit of cementum. 



When, however, the enamel (a) recommences at a point 

 higher up on the crown of the tooth, the cement does not at 

 once cease, but is continued for some little distance, as a 

 thick layer containing lacunee (e), outside the enamel. That 

 this deposit of cement in the midst of the enamel existed 

 from the first formation of the tooth, and was not due to 

 absorption of the enamel and subsequent deposition of 

 osseous structure in its place, is proved by the disturbance 

 in the direction of the dentinal tubes which is noticeable at 

 this point (c?). 



Strongly marked cementum, such as that represented in 

 fig. 1, is seldom met with on the crowns of human teeth ; if, 

 however, vertical sections be made through the crowns of a 

 large number of human molar and premolar teeth, deep 



1 Art. " Odontology," ' Encyclopaedia Brittaniea.' 



2 J. Tomes, 'A System of Dental Surgery,' 1859, p. 272. 



3 'Pathologie dor Zabne,' 1870. 



