ELEMENTARY TISSUES. 



Fig. 21* 



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 mi- 

 nute tubes form gentle and nearly 

 parallel curves, and divide and sub- 

 divide dichotomously, but without 

 much lessening of their calibre until 

 they are approaching their peripheral 

 termination. 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 4T Vo- ^ an i ncn > conta i n nne 

 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. 



