3i8 THE SOLIDS. 



entire surface of the enamel of the tooth ; this would appear, 

 so far as the human tooth is concerned, to be an error. A 

 cellular lamina, however, does really invest the enamel in 

 very young teeth, but this is soon worn away : this layer, 

 however, has nothing to do with the cementum, but is con- 

 sidered by Mr. Tomes to be derived from the inner surface 

 of the membrane of the tooth sac. It may be seen in the 

 teeth of the calf and horse. 



Cementum, is rarely, if ever developed in the pulp cavity, 

 although it has been stated to be so by many observers. 

 The cementum is not unfrequently traversed by tubes, 

 similar to and derived from those of the dentine. 



It will now be very evident that dentine and cementum 

 do not differ essentially from each other, and that both are 

 but modifications of ordinary bone. 



Enamel. Examined with an object-glass of one fourth 

 of an inch focus, the enamel exhibits a fibrous appearance. 



The fibres radiate outwards from the surface of the den- 

 tine, somewhat in the same manner as do the dentinal tubes 

 themselves ; they are simple, short, somewhat attenuated 

 towards either end, and pass towards the margin of the 

 enamel in a waved manner, sometimes decussating, forming 

 plaits or folds, and this occurs especially when the surface 

 of the dentine is concave. Viewed with a glass of the 

 eighth of an inch focus, they present the appearance of 

 elongated and many-sided crystals, and in transverse sec- 

 tions they are seen to be hexagonal or polygonal ; in some 

 instances, however, and especially in the enamel fibres of 

 young teeth, a minute canal may be traced running along 

 each fibre. (See the figures.) 



It is uncertain whether each fibre is constituted of a 

 single cell, or whether several unite to form it : the ap- 

 pearance in some cases of faint transverse markings would 

 render it probable that the latter opinion is the correct 

 one. 



Near the surface of the dentine linear interspaces may 

 sometimes be noticed between the enamel fibres ; with these 

 spaces the tubuli of the dentine frequently communicate, 



