MINUTE STRUCTURE OF TEETH. 



699 



Peirosa. — The Enamel is composed of long prisms, closely resem- 

 bling those of the Prismatic Shell-substance formerly described 

 (§458), but on a 



far more minute Fig. 360. 



scale ; the diameter 

 of the prisms not 

 being more, in 

 Man, than l-5600th 

 of an inch. The 

 length of the prisms 

 corresponds with 

 the thickness of the 

 layer of Enamel ; 

 and 'the two sur- 

 faces of this layer 

 present the ends of 

 the prisms, the 

 form of which 

 Tisually approaches 

 the hexagonal. The 

 course of the 



Enamel - prisms is Transverse Section of Tooth of MyJiobates (Eagle 

 more or less wavy ; Ray) viewed as an opaque object, 



andtheyaremarked 



by numerous transverse stria?, resembling those of the prismatic 

 Shell-substance (§ 458), 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. 361, a) which follows the surface 

 of the dentine in all its inequalities ; and its component prisms 

 are directed at right angles to that surface, their inner extremities 

 resting in slight but regular depressions on the exterior of the den- 

 tine. 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 no continuous layer of enamel over the 

 crown. The purpose of this arrangement is evidently to provide, 

 by the unequal tvear of these three substances, — 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. The 

 Enamel is the least constant of the dental tissues. It is more fre- 

 quently absent than present in the teeth of the Class of Fishes; it 

 is wanting in the entire Order of Ophidia (serpents) among existing 

 Reptiles ; and it forms no part of the Edentata (sloths, &c.) and 

 Cetacea (whales) amongst Mammals. — The Cementum, or C'rusta 



