THE STRUCTURE OF THE ELEMENTARY TISSUES. 



73 



the dentine (the layer is sometimes mentioned as the membmna eboris, 

 from the tenacity with which it clings to the dentine), all are imbedded 

 in a mucoid gelatinous matrix. 



The blood-vessels and nerves enter the pulp through a small opening 

 at the apical extremity of each root. The exact terminations of the 

 nerves are not definitely known. They have never been observed to 

 enter the dentinal tubes, but they are probably connected with the fibrils 

 in those tubes through the intervention of the odontoblasts and deeper 

 layer of cells. No lymphatics have been traced to the pulp. 



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- 



Fig. 73. A. Longitudinal section of a human molar tooth; c, cement; d, dentine; e, enamel; v, 

 pulp cavity (Owen). B. Trausverse section. The letters indicate the same as in A. 



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. 



The enamel and cement are very thin at the neck of the tooth where 

 they come in contact, the cement overlapping the enamel. A thin epi- 

 thelial and horny membrane (enamel cuticle, or Nasmyth's membrane) 

 covers the outer surface of the enamel on unworn teeth. It is formed of 

 short flattened prisms which are the remains of the uncalcified last- 

 formed portions of the enamel prisms. The enamel becomes thicker 

 toward the crown, and the cement toward the lower end or apex of the 

 root. 



I. Dentine or Ivory. 



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 

 resolved into gelatin by boiling. It also contains a trace of fat. The 

 earthy matter is made up chiefly of calcium phosphate, with a small por- 



