366 ANATOMY OF VERTEBRATES. 



scribe two slight primary curves, the first convex towards that 

 end, the second and shorter one concave : these curves in narrow 

 sections from near the open base of the tusk are almost obscured 

 by the strong angular parallel secondary gyrations. The tubes 

 divide dichotomously, at acute angles, and gradually decrease in 

 size as they approach the periphery of the tusk. 



The characteristic appearance of decussating curved striae, with 

 oblique rhomboidal spaces, so conspicuous on transverse sections 

 or fractures of ivory, is due to the refraction of light caused by 

 the parallel secondary gyrations of the tubes above described. 

 The strong contour lines observed in longitudinal sections of 

 ivory, parallel with the cone of the pulp-cavity, and which are 

 circular and concentric when viewed in transverse slices of the 

 tusk, are commonly caused by strata of minute opaque cellules, 

 which are unusually numerous in the interspaces of the tubes 

 throughout the substance of the ivory, and by their very great 

 abundance and larger size in the peripheral layers of cement. 

 The decomposition of the fossil tusks into superimposed conical 

 layers takes place along the strata of the opaque cellules, and 

 directly across the course of the gyrating dentinal tubes. 



By the minuteness and close arrangement of the tubes, and 

 especially by their strongly undulating secondary curves, a 

 tougher and more elastic tissue is produced than results from 

 their disposition in ordinary dentine ; and the modification which 

 distinguishes ' ivory ' is doubtless essential to the due degree of 

 coherence of so large a mass as the elephant's tusk, projecting so 

 far from the supporting socket ; and to be frequently applied in 

 dealing hard blows and thrusts. 



§ 222. Homologies of Teeth. — In Histology tissues differ 

 according to the kinds and degrees of force which they exercise 

 in the living body : some, the nervous and muscular, e.g. are 

 ( active ; ' others, with lower endowments of elasticity, adhesiveness, 

 hardness, &c, may be called ' passive,' and the classes of these 

 tissues are less definite and distinct. In considering the homo- 

 logy of a tooth, in reference to its class of tissue, our view of it 

 must not be restricted to its ordinary conditions in mammalia, 

 where a central pulp-canal radiates a single system of dentinal 

 tubes like the lacunal tubes from a Haversian canal, but should 

 be extended to those less specialised states of tooth in which the 

 body of dentine is traversed by several pulp-canals, either di- 

 chotomising, as in the molar of Orycteropus, vol. i. p. 369, fig. 

 247, or ramifying throughout the dentine, as in the laniariform 

 tooth of Lamna (vol. i. p. 364, fig. 241). 



