TEETH 



1025 



than dentine, are frequently found associated with it ; the former is 

 termed enamel, and the latter cementum or crusta petrosa. The ena,ni>'l 

 is composed of long prisms, closely resembling those of the ; prismatic ' 

 shell-substance formerly described, but on a far more minute scale, the 

 diameter of the prisms not being more in man than ^g^th of an 

 inch. The length of the prisms corresponds with the thickness of 

 the layer of enamel ; and the 

 two surfaces of this layer pre- 

 sent the ends of the prisms, 

 the form of which usually ap- 

 proaches the hexagonal. The 

 course of the enamel prisms is 

 more or less wavy, and they 

 are marked by numerous trans- 

 verse strife, resembling those 



of the prismatic shell-sub- 

 stance, and probably origina- 

 ting in the same cause the 



coalescence of a series of shorter 



prisms to form the lengthened 



prism. In man and in car- 

 nivorous animals the enamel 



covers the crown of the tooth 



only, with a simple cap or 



superficial layer of tolerably 



uniform thickness (fig. 757, a), 



which follows the surface of 



the dentine in all its inequali- 

 ties ; and its component prisms 



are directed at right angles to 



that surface, their inner ex- 



FIG. 756. Transverse 

 Myliobates (eagle 

 opaque object. 



section of tooth 

 ray), viewed as 



of 

 an 



tremities resting in slight but 

 regular depressions on the ex- 

 terior of the dentine. In the 

 teeth of many herbivorous 

 animals, however, the enamel 

 forms (with the cementum) a 

 series of vertical plates which 

 dip down into the substance 

 of the dentine, and present 

 their edges alternately with it 

 at the grinding surface of the 

 tooth ; and there is in such 

 teeth 110 continuous layer of 

 enamel over the crown. This 



arrangement provides by the unequal wear of these three 

 stances (of which the enamel is the hardest, and the cementum the 

 softest) for the constant maintenance of a rough surface, adapted to 

 triturate the tough vegetable substances 011 which these animals feed. 

 Though the enamel is" not always present, it has been shown by Mr. 

 Charles Tomes that the germ from which it is formed always appears 



O -* 



3 u 



Fn;. 757. Vertical section of human molar 

 tooth: a, enamel; b, cementum or crusta 

 petrosa; <; dentine or ivory; <?, osseous 

 excrescence arising from hypertrophy of 

 cementum ; e, pulp-cavity ; /, osseous lacunae 

 at outer part of dentine. 



sub- 



