THE DENTAL TISSUES. 55 



Waldeyer and Hertz doubt the passage of the tubes of the 

 dentine into the enamel ; as Kolliker observes, it is difficult 

 to see how they can doubt it, even after mere observation of 

 a single specimen ; moreover, it is also capable of experi- 

 mental demonstration, for if an acid capable of removing the 

 enamel be applied to one of these sections of marsupial teeth 

 so as to dissolve away the enamel, the freed tubes are left 

 hanging out from the edge of the dentine, thus putting the 

 matter beyond all possibility of doubt, while the develop- 

 ment of the marsupial enamel makes the nature of the con- 

 tents of the tubes quite clear. 



The most marked variation in the structure of enamel, 

 which is on the whole a tissue differing but little in various 

 animals, is met with in the class of fish. 



In the Sargus, or sheep's-head fish, for example, the enamel 

 is penetrated by a system of tubes which are not continued 



FIG. 24 (*). 



out of or derived from the dentine, but belong to the enamel 

 itself. 



The tubes, as seen in the figure, run at right angles to 

 the external surface of the enamel, proceed inwards without 

 branch or bend for some little distance, and then, at about 



(*) Enamel and dentine of the Sheep's-head fish (Sargus ovis). 

 The enamel is penetrated by a system of channels which enter from its 

 free exposed surface, pass in for a certain distance in straight lines, and 

 then abruptly bending at an angle cross one another, and produce a com- 

 plicated pattern in the inner third of the enamel. 



