322 THE SOLIDS. 



longitudinally into two portions, each of which becomes a 

 perfect cell, also provided with a nucleus ; these again divide, 

 but in a contrary direction, viz. transversely ; thus four 

 secondary cells are formed within the cavity of the primary 

 cell, and out of its single nucleus. The number is not, 

 however, limited to four, but each nucleus may give origin 

 to many secondary cells. The primary cells are placed end 

 to end, as are also the secondary cells ; these last elongate 

 considerably, until at length they coalesce, thus forming the 

 tubes of the dentine ; the primary cells remaining as such, 

 and in some adult human teeth being faintly visible. The 

 primary and secondary cells, however, although placed end to 

 end, do not form straight lines, but describe greater and lesser 

 curves, the greater being formed by the primary cells, and 

 the lesser by the secondary ; these are the curvatures to which 

 reference has been made in the description of the tubes of 

 the dentine. 



The views of Mr. Tomes *, although not essentially at 

 variance with those of Professor Owen, yet differ from them 

 in some important particulars. Mr. Tomes describes the 

 primary cells themselves as dividing longitudinally into two 

 or more secondary cells, but not transversely ; subsequent to 

 this division each cell elongates, and at length unites with 

 those above and below it, thus forming the dentinal tubes. 

 Sometimes two cells unite with but a single cell placed beneath 

 it, and it is in this way that the branches of the dentinal tubes 

 are produced. Thus, according to Mr. Tomes, the wall of the 

 primary cell, as well as its nucleus, enters into the formation 

 of the dentinal tubuli, while, according to Professor Owen, 

 the nuclei alone give rise to these tubes, the walls of the 

 parent cells not undergoing elongation, but remaining to con- 

 stitute a considerable portion of the intertubular tissue. 



In the human tooth I have been unable to detect the ex- 

 istence of the primary cells of the dentine. 



Covering the dentine pulp, a thin transparent membrane 

 exists; this, on its outer surface, is marked with numerous 



* Medical Gazette, Lecture V. 



