TEETH 



1025 



opaque object. 



ray), 



tooth of 

 viewed as an 



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



termed enamel, and the latter cementum or crusta petrosa. The enamel 



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 -^Vyth 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 striae, 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- p IG 756. Transverse section of 



nivorous animals the enamel Myliobates (eagle 



covers the crown of the tooth 



only, with a simple cap or 



superficial layer of tolerably 



uniform thickness (fig. 757, ), 



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- 



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 pio _ 757> _ vertical section of human molar 

 their edges alternately with it tooth: a, enamel; &, cementum or crusta 

 at the grinding surface of the petrosa; c, dentine or ivory; d osseous 

 tooth; and there is in such 

 teeth no continuous layer of 

 enamel over the crown. This 

 arrangement provides by the unequal ivear of these three sub- 

 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 on 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 



3 u 



a t outer part of dentine. 



