TEETH. 



61 



earthy matter is made up chiefly of phosphate of lime, 

 with a small portion of the carbonate, and traces of some 

 other salts. 



Under the microscope, dentine is 

 seen to be finely channelled by a mul- 

 titude of fine tubes, which, by their 

 inner ends, communicate with the 

 pulp-cavity, and by their outer extre- 

 mities come into contact with the under 

 part of the enamel and cement, and 

 sometimes even penetrate them for a 

 greater or less distance. In their 

 course from the pulp-cavity to the 

 surface of the dentine, these minute 

 tubes form gentle and nearly parallel 

 curves, and divide and subdivide 

 dichotomously, but without much 

 lessening of their calibre until they 

 are approaching their peripheral ter- 

 mination. From their sides proceed 

 other exceedingly minute secondary 

 canals, which extend into the dentine 

 between the tubules. 



The tubules of the dentine, the 

 average diameter of which at their 

 inner and larger extremity is 45^ of an inch, contain fine 

 prolongations from the tooth-pulp which give the dentine 

 a certain faint sensitiveness under ordinary circumstances, 

 and, without doubt, have to do also with its nutrition. 



* Fig. 21. Magnified Longitudinal Section of a Bicuspid Tooth 

 (after Eetzius). I, the ivory or dentine, showing the direction and 

 primary curves of the dental tubuli; 2, the pulp-cavity, with the 

 small apertures of the tubuli into it ; 3, the cement or crusta petrosa, 

 covering the fang as high as the border of the enamel at the neck, 

 exhibiting lacunae ; 4, the enamel resting on the dentine ; this has been 

 worn away by use from the upper part. 



