464 HUMAN DENTITION. 



I 



observable in the two views of the cells, that surface of the cell 

 which is next the periphery of the tooth is shown to be more 

 rounded and convex, than the other surfaces. Viewed by transmitted 

 light, the cell-boundaries are usually opake ; but by a slight alteration 

 of focus the dark lines become clearer and brighter than the sur- 

 rounding substance : in parts of the dentine near the periphery, the 

 boundaries are occasionally seen to be irregularly widened ; as if 

 contiguous cells were separated by depositions of Ume-salts in a 

 sub-granular state : these thickened and less regular boundaries are 

 always opake, and when viewed by reflected light are white. As the 

 dentine approaches the pulp-cavity the indications of the cells 

 gradually decrease in size, and their boundaries usually become 

 fainter and less complete ; viewed in longitudinal section the peri- 

 pheral margin is the last to disappear. 



After noticing the cells and tubes of the dentine, the third 

 and least important appearance, which relates rather to the course of 

 formation than to the essential structure of the dentine, is that 

 produced by the lines, which from their paralleHsm with the vertical 

 outline of the crown I have termed ' contour-Hnes.' These are 

 the lines, or striae of the dentine, which, in sections of the larger 

 teeth of many of the inferior MammaUa, are visible by the naked 

 eye, through the action of a short parallel bend of the dentinal tubes, 

 along the course of the contour line, upon the rays of light. These 

 lines are not equally conspicuous in every tooth ; I have usually 

 found them most so in the molars of the human subject, where, 

 without being regularly equi-distant, they have presented intervals 

 of about yjoth of an inch, commencing at thrice that distance from 

 the periphery of the dentine(] ). 



The enamel of the human teeth (2) consists of long and slender 

 solid, prismatic, for the most part hexagonal, fibres of phosphate, 

 carbonate, and fluat of lime, in the proportions mentioned in the 

 * Introduction' : they are essentially the contents of extremely delicate 

 membranous tubes, originally sub-divided into minute depressed 



(1) PI. 122, fig. 7, I. The dentine in fossil teeth commonly breaks up along these lines 

 into conical or superimposed strata. 



(2) PI. 122, fig, 1—7, e. 



