316 THE SOLIDS. 



manner and form distinct loops, at others the terminal 

 branches pass out of the dentinal substance, and extend either 

 into the cementum or the enamel ; this extension into the 

 former is a very frequent occurrence. 



For illustrations of these several modifications, the majority 

 of which have been pointed out by Mr. Tomes in his excellent 

 lectures *, see the figures. 



The surface of the dentine presents many elevations and 

 depressions : to these the enamel is accurately adapted ; it 

 also exhibits the hexagonal impressions of the enamel fibres. 



The substance of the dentine is seen also occasionally to 

 be traversed with canals for blood-vessels, analogous to the 

 Haversian canals of bone. 



The pulp cavity of the teeth _of old persons frequently be- 

 comes filled up, and even obliterated, by a secondary formation 

 of dentinal substance, and which may be called the secondary 

 dentine. This dentine results from the ossification of the 

 pulp by the vessels of which it is traversed, and from the 

 margins of the canals containing which the dentinal tubes 

 proceed in a radiate manner. (See the figures.) 



Mr. Nasmyth regarded this secondary dentinal formation 

 as distinct from the other structures of the tooth, and called 

 it the fourth dentinal constituent. 



The dentinal tubes form but one element of the dentine ; 

 the other is the intertubular substance. 



This is described by Mr. Nasmyth as constituted of elon- 

 gated cells, in the form of bricks placed end to end, and a 

 tier of which exists between every two tubes : Henle, on 

 the other hand, declares it to be fibrous. It would appear 

 not to present any regular tissue, but to be simply granular. 



Occasionally, I have encountered in it globules of various 

 sizes refracting the light strongly, and presenting the appear- 

 ances of oil or fat vesicles. It has occurred to me that these 

 might be fat cells which had become included in consequence 

 of the ossification of the pulp, and which always contains a 

 greater or less quantity of fat cells. (See figure.) 



* See Lectures in Medical Gazette. 



