790 



THE TEETH. 



clusters of the fibres down to the surface of the dentine ; and other much 

 larger and more evident fissures are often observed leading down from the 

 depressions or crevices between the cusps of the molar and premolar teeth. 

 The surface of the enamel, especially in the milk-teeth, is marked by 

 transverse ridges, which may be distinguished with a common magnifying 

 glass. 



Fig. 549. 



Fig. 549. ENAMEL FIBRES (from Kolliker). s f 



A, fragments and single fibres of the enamel, isolated by the action of hydrochloric acid. 



B, surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 



The enamel-fibres have the form of solid hexagonal prisms. Their dia- 

 meter varies slightly, and is ordinarily about j^o^th of an inch. They are 

 marked at small intervals by dark transverse lines. According to Tomes, 

 the fibre is not in all cases solid, but has occasionally an extremely minute 

 cavity in part or in the whole of its length, which is best seen in newly- 

 developed enamel, but is also visible in adult teeth. The inner ends of the 

 prisms are implanted, as it were, into the minute hexagonal depressions 

 found on the surface of the dentine, whilst the outer ends, somewhat larger 

 in diameter, are free, and present, when examined with a high magnifying 

 power, a tesselated appearance. 



When submitted to the action of dilute acids, the enamel is almost 

 entirely dissolved, and leaves scarcely any discernible traces of animal 

 matter. Near the deep surface this is rather more abundant, according to 

 the observations of Retains, who conceived that it there aided in fixing the 

 enamel fibres. By the action of an acid, the enamel of newly formed or 

 still growing teeth may be broken up, and its structural elements more 

 easily distinguished. The prisms are then found to have interposed between 

 them a delicate membranous structure, forming sheaths in which the cal- 

 careous matter is deposited. As this latter accumulates, the membranous 

 structure becomes almost or entirely obliterated, and the now earthy prisms 

 are inseparably consolidated. Each membranous sheath, according to Tomes, 

 contains a line of granular cells or masses, arranged in single series like the 

 sarcous elements in muscular fibres, and thus occasioning the transverse 

 markings. 



