56 HAND-BOOK OF PHYSIOLOGY. 



At the neck of the tooth, where the enamel and cement come into 

 contact, each is reduced to an exceedingly thin layer. The covering of 

 enamel becomes thicker as we approach the crown, and the cement as we 

 approach the lower end or apex of the fang. 



L 



Chemical composition. Dentine or ivory in chemical composition 

 closely resembles bone. It contains, however, rather less animal matter; 

 the proportion in a hundred parts being about twenty-eight animal to 

 seventy-two of earthy. The former, like the animal matter of bone, may 

 be resolved into gelatin by boiling. The earthy matter is made up chiefly 

 of calcium phosphate, with a small portion of the carbonate, and traces 

 of calcium fluoride and magnesium phosphate. 



Structure. Under the microscope dentine is seen to be finely chan- 

 neled by a multitude of delicate tubes, which, by their inner ends, com- 



FIG. 60. Section of a portion of the dentine and cement from the middle of the root of an incisor 

 tooth, a, dental tubuli ramifying and terminating, some of them in the interglobular spaces b and c, 

 which somewhat resemble bone lacunae; d, inner layer of the cement with numerous closely set 

 canaliculi; e, outer layer of cement;/, lacunae; g, canaliculi. x 350. (Kolliker.) 



municate with the pulp-cavity, and by their outer extremities come into 

 contact with the under part of the enamel and cement and sometimes 

 even penetrate them for a greater or less distance (Fig. 60). 



In their course from the pulp-cavity to the surface of the dentine, the 

 minute 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, and anastomose with 

 each other. The tubules of the dentine, the average diameter of which 

 at their inner and larger extremity is T3 V<r f an inch* contain fine pro- 

 longations 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. These prolongations from the tooth-pulp are 

 really processes of the dentine-cells or odontollasts which are branched cells 

 lining the pulp-cavity; the relation of these processes to the tubules in 



