CH. V.] DENTINE 69 



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

 ing 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. No lymphatics have been seen in 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 

 sheathing the portion of dentine which is beneath the level of the 

 gum, is a layer of true bone, Called the cement or crusta petrosa. 



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

 contact, each is reduced to an exceedingly thin layer ; here the cement 

 overlaps the enamel, and is prolonged over it. On the surface of the 

 crown of the tooth, when it first comes through the jaw, is a thin 

 membrane called Nasmyth's membrane, or the cuticle of the tooth. 

 The covering of enamel becomes thicker towards the crown, and the 

 cement towards the lower end or apex of the root. 



Dentine or Ivory. 



Dentine closaly resembles bone in chemical composition. It con- 

 tains, however, only 10 per cent, of water. The proportion in a 

 hundred parts of the solids is about twenty-eight animal to seventy- 

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

 may be converted into gelatin by boiling. It also contains a trace of 

 fat. 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. 



Under the microscope dentine is seen to be finely channelled 

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

 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 (figs. 95, 

 97). The matrix in which these tubes lie is composed of " a reticulum 

 of fine fibres of connective tissue modified by calcification, and where 

 that process is complete, entirely hidden by the densely deposited lime 

 salts " (Mummery). 



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

 tubes form gentle and nearly parallel curves, and divide and subdivide 

 dichotomously, but without much lessening of their calibre until they 

 approach 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 T -Vo of an 

 inch, contain fine prolongations from the tooth-pulp, which give the 

 dentine a certain faint sensitiveness under ordinary circumstances, and, 



