THE TEETH. 



41 



even upon the neck of the tooth, by a thin layer of enamel, 

 and when this ceases by cement. 



The dentine consists of a matrix and of a multitude of canals 

 which traverse it, the dentinal tubules or canals. In the recent 

 tooth, the former is, even in the finest sections, quite homo- 

 geneous, without the slightest trace of cells, fibres, or other 

 elements. After the extraction of the calcareous salts from 

 the dentine, it exhibits, however, a great tendency to tear up 

 into coarse fibres, parallel to the dentinal canals ; from these, 

 finer fibres of 0*002 — 0003'" may be detached, their irregular 

 form, however, shows them to be artificial products, and in 

 fact they owe their existence simply to the circumstance, that the 

 dentinal canals run close together and parallel to one another 

 through the dentine. The matrix exists in all parts of the 

 dentine, but not everywhere to the same amount. In general 

 there is less of it in the crown than in the root, and in the 

 neighbourhood of the pulp cavity than in that of the enamel 

 and cement. 



The dentinal canals (fig. 184, 187) are microscopic tu- 

 bules of 0-0006— O-OOl'" (in the root some reach 0-002'"), 

 which commence by open 

 mouths upon the wall of the 

 pulp cavity, and traverse 

 the whole thickness of the 

 dentine to the cement and 

 enamel. Each canal has 

 a special wall, rather less 

 in thickness than its dia- 

 meter, which can only be observed in transverse sections, 

 (and then not always), as a narrow, yellowish ring sur- 

 rounding the cavity; in longitudinal sections, on the other hand, 

 it is almost entirely invisible. During life the canals 

 contain a clear fluid and they cannot therefore readily be de- 

 tected in recent preparations ; it is different in dry sections, 

 when they become filled with air, and appear separately as black 

 lines by transmitted light, and by reflected, as silvery threads. 

 On account of the immense numbers of the dentinal canals, so 

 great in some situations that their walls are almost in contact, 



Fig. 184. Transverse section of dentinal canals as they are commonly seen, x 450 : 

 a, canals very close together ; b, more dispersed. 



