172 



DIGESTION. 



[CHAP. xxin. 



The rods of enamel are about 

 Fig. 152. 



rests on the dentine, which presents a number of minute depressions 

 for the reception of the deep ends of the vertical rods, a number 

 of which rest in each ; and the superficial surface, though said to be 

 at first coated with a thin film of osseous tissue, is afterwards ren- 

 dered bare by the earliest movements of attrition in masticating the 

 food, and then becomes the free surface of the crown of the tooth. 



th of an inch in diameter, 

 and they pursue a more 

 or less meandering course, 

 which must augment their 

 power of resisting external 

 force. It is evident that their 

 vertical position admirably 

 adapts them to sustain pres- 

 sure, and withstand the ef- 

 fects of force directed upon 

 the surface of the tooth; 

 while, at the same time, the 

 interstices or chinks inter- 



A. Vertical section of the enamel, shewing the fibres, V6nin g between them, prin- 

 W B h SbresTflhe'namel, seen endwise. ci P a % at tneir an g leS f J UX ~ 



Magnmed 350 diameters. From Retzius. taposition, are arranged in 



the most suitable manner for permeation by the fluids derived from 

 the subjacent dentinal tubuli. These tubuli, indeed, may be 

 seen to communicate directly with the interstitial passages of the 

 enamel. The enamel rods are further marked, at pretty close 

 and regular intervals, by cross lines, which, however, are far from 

 constant, and of doubtful nature : some suppose them explained 

 by the process of development. The enamel rods are connected 

 together by some remnant of the original organic matrix in which 

 their earthy portion was at first deposited, and which is represented 

 by the dark lines or chinks which appear to bound and isolate the 

 rods. As the rods are placed vertically on the surface of the den- 

 tine, which is not an even one, they are not everywhere parallel 

 or of equal length, but are truncated where they abut against each 

 other over a hollow, and in such parts are most liable to decay. 

 In many parts, however, near the dentine their vertical position 

 seems disordered ; they are curiously contorted, and neighbouring 

 series of them are variously inclined, so as to lean against one 

 another, while the same rods nearer the surface assume an upright 

 and parallel course. The enamel on a vertical section further 

 shews dark markings running obliquely across the fibres (fig. 150, 



