THE TEETH. 469 



brittle than either the cement or hone, but, on the other hand, yields in 

 these qualities to the enamel. With the exception of a very small spot 

 in the root, the dentine forms the sole boundary of the pulp cavity, and 

 in an uninjured tooth it is never exposed, inasmuch as it is covered, 

 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 for- 

 mer is, even in the finest sections, quite homogeneous, 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 0-002-0-003 of a line 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 denti- 

 nal canals run close together arid parallel to one another through the 

 dentine. The matrix exists in all parts of the dentine, but not every- 

 where to the same amount. In general there is less of it in the crown 

 than in the root, and in the neighborhood of the pulp cavity than in that 

 of the enamel and cement. 



The dentinal canals (Figs. 184, 187) are microscopic tubules of 0-0006- 

 0-001 of a line (in the root some reach 0-002 of a line), 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 diameter, 

 which can only be observed in trans- 

 verse sections (and then not al- 

 ways), as a narrow, yellowish ring surrounding the cavity; in longi- 

 tudinal sections, on the other hand, it is almost entirely invisible. 

 During life the canals contain a clear fluid and they cannot therefore 

 readily be detected 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 situa- 

 tions that their walls are almost in contact, dry sections appear milk- 

 white, and if not very thin, are quite unfit for microscopic investigation, 

 unless the air has been previously expelled from the canals by any clear 

 and not viscid fluid. 



FIG. 184. Transverse section of dentinal canals as they are commonly seen, magnified 

 450 diameters : a, canals very close together ; 6, more dispersed. 



