52 



ELEMENTARY TISSUES. 



FIG. 21. 



bone. It contains, however, rather less animal matter ; the 



proportion in 100 parts being 

 about 28 of animal matter to 

 72 of earthy. The former, like 

 the animal matter of bone, may 

 be resolved into gelatin by boil- 

 ing. The earthy matter is made 

 up chiefly of phosphate of lime, 

 with a small portion of the car- 

 bonate, and traces of some other 

 salts. 



Under the microscope, den- 

 tine is seen to be finely chan- 

 nelled by a multitude of fine 

 tubes, which, by their inner 

 ends, communicate with the 

 pulp-cavity, and by their outer 

 extremities come into contact 

 with the under part of the en- 

 amel and cement, and some- 

 times 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 dichotom- 

 ously, but without much lessen- 

 ing of their calibre until they 

 are approaching their peripheral 

 termination. From their sides 

 proceed other exceedingly mi- 

 nute secondary canals, which 

 extend into the dentine between 

 the tubules. 



The tubules of the dentine, 



Magnified Longitudinal Section of a 

 Bicuspid Tooth (after Retzius) 1, the 

 ivory or dentine, showing the direc- 

 tion 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, cover- 

 ing the fang as high as the border of 

 the enamel at the neck, exhibiting 



lacuna? ; 4, the enamel resting on the , , f 



dentine; this has been worn away by the average diameter of which 



use from the upper part. at their inner and larger ex- 



tremity is 4^0 o f an i ncn > con " 



tain fine prolongations from the tooth-pulp, which give the den- 

 tine a certain faint sensitiveness under ordinary circumstances, 

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



The enamel, which is by far the hardest portion of a tooth, 

 is composed, chemically, of the same elements that enter into 

 the composition of dentine and bone. Its animal matter, how- 

 ever, amounts only to about 2 or 3 per cent. 



