224 HANDBOOK OF PHYSIOLOGY. 



A layer of very hard calcareous matter, the enamel, caps that part of 

 the dentine which projects beyond the level of the gum. ; while sheath- 

 ing the portion of dentine which is beneath the level of the gum, is a 

 layer of true bone, called the cement or crusta petrosa. 



Fio. 166. A. Longitudinal section of a human molar tooth: c, cement; d, dentine; e, enamel; v, 

 pulp cavity. (Owen.) 



B. Transverse section. The letters indicate the same as in A. 



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 towards the crown, and the cement towards the 

 lower end or apex of the fang. 



I. Dentine. 



Chemical composition. Dentine closely resembles bone in chemical 

 composition. It contains, however, rather less animal matter ; the pro- 

 portion in a hundred parts being about twenty-eight animal to seventy- 

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

 solved 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- 

 nelled by a multitude of delicate 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 enamel and cement, and some- 

 times even penetrate them for a greater or less distance (Fig. 168). 



In their course from the pulp-cavity to the surface, the minute tubes 

 form gentle and nearly parallel curves and divide and subdivide dicho- 

 tomously, but without much lessening of their calibre until they are ap- 

 proaching their peripheral termination. 



From their sides proceed other exceedingly minute secondary canals, 



