260 THIS MICROSCOPE AND ITS REVELATIONS. 



the prisms, the form of which usually approaches the hexagonal. The 

 course of the enamel-prisms is more or less wavy; and they are marked 

 by numerous transverse striae, resembling those of the prismatic shell- 

 substance, and probably originating in the same cause, the coalescence 

 of a series of shorter prisms to form the lengthened prism. In Man and 

 in Carnivorous animals the enamel covers the crown of the tooth only, 

 with a simple cap or superficial layer of tolerably uniform thickness 

 (Fig. 446, 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 extremities 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 alter- 

 nately with it, at the grinding surface of the tooth; and there is in such 

 teeth no continuous layer of enamel over the crown. This arrangement 

 provides, by the unequal wear of these three substances (of which the 

 enamel is the hardest, and the cementum the softest), for the constant 



FIG. 445. FIG. 446. 



A 



Transverse section of Tooth of Myliobates Vertical section of Human Molar Tooth: 



(Eagle Ray) viewed as an opaque object. a, enamel; b, cementum or crusta petrosa; 



c, dentine or ivory ; d, osseous excrescence, 

 arising from hypertrophy of cementum ; e, 

 pulp-cavity;/, osseous lacunae at outer part 

 of dentine. 



maintenance of a rough surface, adapted to triturate the tough vegetable 

 substances on which these animals feed. The enamel is the least constant 

 of the dental tissues. It is more frequently absent than present in the 

 teeth of Fishes; it is entirely wanting in the teeth of Serpents; audit 

 forms no part of those of the Edentata 1 (sloths, etc.) and Cetacea 

 (whales) among Mammals. The cementum, or crnsta petrosa, has the 

 characters of true bone; possessing its distinctive stellate, lacunae and 

 radiating canaliculi. Where it exists in small amount, we do not find it 

 traversed by medullary canals; but, like dentine, it is occasionally 

 furnished with them/ and thus resembles bone in every particular. 

 These medullary canals enter its substance from the exterior of the tooth, 



1 It has been shown by Mr. Charles Tomes, however, that the 'enamel organ' 

 is originally present within the tooth-capsule of the Armadillo, though it under- 

 goes an early degeneration; a fact of no little interest in connection with the 

 general doctrine of ' Descent with modification.' 



