54 THE CONNECTIVE TISSUES [CH. V. 



by branching into fine fibrillre which 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 closely 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, 



9 c 



FIG. 66. Section of a portion of the dentine and cement from the middle of the root of an incisor tooth, 

 a, Dentinal tubules ramifying and terminating, some of them in the interglobular spaces 5 and c ; d, 

 inner layer of the cement with numerous closely set canaliculi ; e, outer layer of cement ; /, lacunrc ; 

 g, canaliculi. x 350. (Kolliker.) 



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. 66, 

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

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



