VERTEBRATED ANIMALS. 



259 



division of the tooth, the individual tubes sometimes giving-off lateral 

 branches, whilst in other instances their trunks bifurcate. This arrange- 

 ment is peculiarly well displayed, when sections of teeth constructed upon 

 this type are viewed as opaque objects (Fig. 445). In the teeth of the 

 higher Vertebrata, however, we usually find the centre excavated into a 

 single cavity (Fig. 446), and the remainder destitute of vascular canals; 

 but there are intermediate cases (as in the teeth of the great fossil 

 Sloths) in which the inner portion of the dentine is traversed by pro- 

 longations of this cavity, conveying blood-vessels, which do not pass into 

 the exterior layers. The tubuli of the t 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 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; 



Fto. 443. 



FIG. 444. 



Fig. 443. Perpendicular section of Tooth of 

 Lamna, moderately enlarged, showing network 

 of medullary canals. 



Fig. 443. Transverse section of portion of 

 Tooth of Pristis, more highly magnified, show- 

 ing orifices of medullary canals, with systems 

 of radiating and inosculating tubuli. 



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 curva- 

 tures ' probably indicate, in dentine, as in the Crab's shell ( 613), suc- 

 cessive 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. 



656, 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 dentine; the former is termed 

 enamel; and the latter cementum or crusta petrota. The enamel is com- 

 posed of long prisms, closely resembling those of the ' prismatic ' Shell- 

 substance formerly described ( 563), but on a far more minute scale; 

 the diameter of the prisms 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 



