608 



MINUTE STRUCTURE OF TEETH. 



cases (as in the teeth of the great fossil Sloths) in which the inner 

 portion of the Dentine is traversed by prolongations of this cavity, 

 conveying Blood-vessels, which do not pass into the exterior layers. 

 The Tubuli of the ' non-vascular' Dentine, which exists by itself in 

 the teeth of nearly all Mammalia, and which in the Elephant is 



Fie. 358. 



Fig. 359. 



Fig. 358. Perpendicular section of Tooth of Lamna, moderately 

 enlarged, showing network of Med\;llary Canals. 



Fig. 359. Transverse section of portion of Tooth of Pristis, 

 more highly magnified, showing orifices of Medullary Canals, 

 with systems of radiating and inosculating Tubuli. 



known as 'ivory,' all radiate from the central cavity, and pass 

 towards the surface of the tooth in a nearly parallel course. Their 

 diameter at their largest part averages 1-10, 000th of an inch; 

 their smallest branches are immeasurably fine. The Tubuli in 

 their course present greater and lesser undulations ; the former 

 are few in number ; but the latter are numerous, and as they 

 occur at the same part of the course of several contiguous tubes, 

 they give rise to the appearance of lines concentric with the centre 

 of radiation. These ' secondary curvatures ' probably indicate, 

 in Dentine, as in the Crab's She)] (§ 507), successive stages of 

 calcification. — The Tubuli are occupied, during the life of the 

 Tooth, by delicate threads of protoplasmic substance, extending 

 into them from the central pulp (§ 540). 



546. In the Teeth of Man and most other Mammals, and in 

 those of many Reptiles and some Fishes, we find two other sub- 

 stances, one of them harder, and the other softer, than Dentine ; 

 the former is termed Enamel ; and the latter Cement um or Crusta 



