190 



ANIMAL PHYSIOLOGY. 



[LESSON 53. 



is given (Fig. 304); here the facets are four, five, or six-sided, with 

 a nucleated spot apparent in each of them. 



820. The isolated prisms are also given in Fig. 305 ; the trans- 

 verse lines serve to give them a jointed appearance ; the sides, too, 

 are remarkable for their irregularity. 



821. If the mode by which the several prisms are connected to 



FIG. 804. 



FIG. "305. 



Facets of the enamel, human. 



Prisms of enamel, human. 



FIG. 306. 



each other, and enabled to form a solid tissue, be obscure, there can 

 be no doubt of the manner in which the dentine becomes firmly con- 

 nected to the enamel. The dentinal tubuli, as they approach the 

 enamel, divide and form Y-like terminations, which (in the molar 



teeth) penetrate the enamel, 

 and thus form a bond of un- 

 ion; but in human canine 

 teeth, a much more efficient 

 plan is in operation, as shown 

 in Fig. 306, which repre- 

 sents the junction of the den- 

 tine and enamel in a human 

 canine tooth. Tubes of the 

 dentine (a, a) having entered 

 the enamel, become tortu- 

 ous, and greatly increase in 

 size, especially as they ap- 

 proach their terminations ; 

 they form a series of wedges, or dowells, which renders the contact 

 so firm that no earthly power can separate tissues so joined. Strike 

 a tooth on an anvil with a hammer, the blow will break the tooth in 

 all manner of directions, but no mechanical skill can separate the 



Junction of dentine and enamel, human 

 Canine Tooth. 



