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 

 lacunse 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 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 



PORTION OF ENAMEL (highly magni- transverse section (fig. 19): and 

 fied), showing its component prisms. 



their course is usually more or 



less wavy. In teeth which have to sustain an extraordinary 

 amount of compression (as is especially the case with those of 



