STRUCTURE OF DENTINE. 469 



vincing myself of this fact. No conclusion can be drawn with 

 positive certainty from sections, since the slightest deviation from 

 parallelism in the surfaces may easily produce deceptive appearances. 

 So, again, fissures in the enamel, and inequalities of the adjacent 

 surfaces of the dentine and enamel, might easily lead to the view 

 supported by Tomes. The question can only be determined by the 

 examination of young teeth in process of development ; but I have 

 never been able to discover anything of the kind. Intervening 

 between the dentine and the cement is a considerable quantity of the 

 already mentioned interglobular substance, and the greater number 

 of the dentinal tubuli open into its irregular spaces. These again 

 are continuous with the lacunae of the cement by means of fine 

 canaliculi. The tubuli may be followed quite to the free surface of 

 the masticatory surface of the incisor teeth of the Rodents, where 

 the dentine is freely exposed ; but it appears to me that in the 

 peripheral portions of these tubules the dentinal fibres are atrophied. 



If we now proceed to consider the dentinal fibres with more 

 minuteness, no further reference to their course and direction 

 is needed, since these are determined by that of the tubules, 

 which have already been sufficiently described. At the same 

 time it is not easy to decide whether the fibres are present in 

 the finest peripheric ramifications of the tubules. In young 

 teeth this is certainly the case, but in those that are older 

 atrophy of the fibres appears to be concurrent with oblitera- 

 tion of the canaliculi. We may seek in vain, even in young 

 dentinal fibres, for rudiments of nuclei, although both the 

 history of their development and several pathological appear- 

 ances (as for instance those accompanying caries) might lead 

 us to expect their presence. The fibres easily stain with 

 carmine. They possess a remarkable degree of extensibility, 

 so that, especially in young teeth, the dentinal cells may be 

 separated to a considerable distance from the dentine without 

 rupture of the processes, which then appear like harp strings 

 stretched across the interval. Salter (51), in recently describing 

 the fibres as tubules, because, when dry, they appear to contain 

 air vesicles, and exhibit a dark central point on section, has 

 probably had the dentinal sheaths under observation. The 

 fibres are really completely solid and homogeneous. 



There are some remarkable deviations from the above-described 



