DENTINE OR IVORY. 239 



ering of enamel, the crusta petrosa, and pulp. Its color 

 is a yellowish white, and presents when broken a fibrous 

 appearance. It is harder than bone. Chemical analysis 

 makes ivory to consist, in 100 parts, of 



Phosphate of lime, 61.95 



Fluate of lime, 2.10 



Carbonate of lime, 5.30 



Phosphate of magnesia, 1.25 



Soda and chloride of sodium, 1.40 



Cartilage and water, 28.00 



According to Mr. Nasmyth, ivory presents three varieties. 

 The first, consisting of a " regular series of fibres and cells," 

 called Jibro-cellular, and regarded as the most perfect kind 

 of ivory, forms the greater portion of the teeth of man, and 

 is found to strongly characterize both classes of the mam- 

 malia and reptilia. The second variety of ivory presents 

 vertical canals traversing it, as seen particularly in the 

 teeth of fish, and is called canalicular. The third variety, 

 from exhibiting, like bone, little corpuscular bodies scat- 

 tered through its substance, receives the name of corpus- 

 cular ivory. Specimens of this latter are noticed in the 

 teeth of the walrus, sloth, &c., and is stated to exist in the 

 human tooth, sometimes, when diseased. 



The structure of the dentine, and its relation to the pulp, 

 are seen in Fig. 61, A, after Ketzius; here the fibres are rep- 

 resented as tubular, the tubes or dental canals opening by 

 circular orifices in the pulp cavity, from which they traverse 

 the body of the tooth, in a curvilinear direction, to end in 

 cul de sacs at the outer margin of the dentine, or at the 

 enamel. These tubes are represented as having distinct 

 parietes, branching in their course, some bifurcating at 

 their termination, others at their middle, and containing a 

 serous fluid, which is supposed to be intended for the nour- 

 ishment of the tooth, by imbibition. It appears from Mr. 

 Nasmyth's later experiments that the structure of dentine 

 is, like that of the pulp, essentially cellular and. fibrous that 

 is, consisting of cells and fibres and that these cells assume 



