82 A MANUAL OF DENTAL ANATOMY. 



systems of dentinal tubes pass from them in various direc- 

 tions without producing the symmetrical patterns which 

 characterise the upper part of the crown. 



FIG. 41 0). 



When the tooth comes into use and its immediate surface- 

 gets worn off, the ends of the perpendicular pulp channels 

 would be laid open, were it not that they become blocked 

 "by the deposition of a transparent homogeneous tissue 

 within them, analogous to the similar tissue which closes 

 Haversian canals of an antler about to be shed. 



Such is an example of plici-dentine in a simple form, in 

 which the tooth might be said to be built up of a series of 

 small parallel denticles; and a similar structure is pre- 

 sented by the rostral teeth of the saw-fish, and by the teeth 

 of the Orycteropus or Cape ant-eater. 



Vaso-dentine. In the dentine of human teeth it occa- 

 sionally happens that a larger canal is found, having no 

 clear relation to the course of the dentinal tubes, which it 

 crosses at various angles ; this larger canal contained the 



( J ) Transverse section of the dentine of Myliobates. 



