68 STRUCTURE OF TEETH. 
course, they appear to be nearly parallel (fig. 18), though 
usually more or less wavy. They occasionally divide into 
two branches, which continue to run, at a little distance from 
one another, in the same parallel direction; and they also 
frequently give off small lateral branches, which again send 
off smaller ones. In some animals the tubuli may be traced 
at their extremities into minute cavities analogous to the 
lacunze of bone ; and the lateral branchlets also occasionally 
terminate in similar cavities. Thus the whole tooth may be 
likened, in some degree, to a single Haversian system in 
bone ; the central cavity, which is lined by a vascular mem- | 
brane, representing the Haversian canal, while the radiating - 
tubuli of the former correspond with the radiating canaliculi 
of the latter ; the chief difference lying in the absence of 
lacunz along the course of the radiating tubes. In a large - 
proportion of Fishes, however, there is no single central cavity, — 
but the whole tooth is traversed by a system of medullary — 
canals, not only resembling the Haversian, but actually con-— 
tinuous with those of the bone on which the tooth is im-— 
planted ; and as each of these is the centre of a distinct — 
system of radiating tubuli, the resemblance of their dentine 
to bone.is very close. .A somewhat similar condition of the 
dentine (obviously a lower or less specialized form of this — 
substance) presents itself in certain Reptiles and Mammals.— — 
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, which is called 
| Enamel, consists of long pris- 
, matic cells, which pass from one 
|| surface to the other of the thin | 
layer formed by this substance 
over the crown, or sometimes in — 
the interior of the tooth (§ 182). 
These prisms are usually hex-— 
Fig. 19. agonal in form, as is seen in 
NAMEL (highly magni- ; p)\» 
Pi Fete Ue contenant eats ae satbgh vs fala: } 
less wavy. In teeth which have to sustain an extraordinary | 
amount of compression (as is especially the case with those of | 
