STRUCTURE OF TEETH. 589 



of the tooth, the individual tubes sometimes giving off lateral 

 branches, whilst in other in- 

 stances their trunks bifurcate. FIG. 304. 

 This arrangement is pecu- 

 liarly well displayed, when 

 sections of teeth constructed 

 upon this type are viewed as 

 opaque objects (Fig. 304). In 

 the teeth of the higher Ver- 

 tebrata, however, we usually 

 find the centre excavated into 

 a single cavity (Fig. 305), and 

 the remainder destitute of 

 vascular canals ; bat there are 

 intermediate cases (as in the 

 teeth of the ^reat fossil Sloths) 



1 i ,i , n Iransverse Section of Tooth of Myliobates (Eagle 



in which the inner portion ot Ray) view ed as an opaque object. 



the dentine is traversed by 



prolongations of this cavity, conveying bloodvessels, 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 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 l-10,000th of an inch; their smallest branches are im- 

 measurably fine. It is impossible that even the largest of them 

 can receive blood, as their diameter is, far less than that of the 

 blood-discs; but it is probable that, like the canaliculi of bone, 

 they may absorb nutrient matter from, the vascular surface upon 

 which their inner extremities open. 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 ot 

 radiation. These secondary curvatures probably indicate, in 

 dentine, as in the crab's shell ( 374) successive stages of calcifi- 

 cation. 



407. In the teeth of Man and most other Mammals, and in 

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

 substances, one of them harder, and the other softer, than den- 

 tine ; the former is termed Enamel ; and the latter Cementum or 

 Crusta Petrosa. The Enamel is composed of long prismatic cells, 

 closely resembling those of the prismatic shell-substance for- 

 merly described (f 336), but on a far more minute scale ; the 

 diameter of the cells not being more, in Man, than l-5600th 

 of an inch. The length of the prisms corresponds with the 

 thickness of the layer of enamel ; and the two surfaces of this 

 layer present the ends of the prisms, the form of which usually 

 approaches the hexagonal. The course of the enamel-prisms is 



