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, wliich again send 
off smaller ones. In some animals the tubuli may be traced 
at their extremities into minute cavities analogous to the 
lacunee 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 
lacunae 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 Eeptiles and Mammals.— 
In the Teeth of Man and most other Mammals, and in those 
of many Eep tiles 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¬ 
agonal in form, as is seen in 
Fig. 19. 
less wavy. In teeth which have to sustain an extraordinary 
amount of compression (as is especially the case with those of 
