////: IHI;I-:STI\-I-: .\rr.\n.\ri > /\ i/.n/u.i/./.i. 



its mass arc dejKwited the saline molecules which give tho dentine its bony 

 consistency. (These molecules sire deposited in lamellae, concentric with 

 tho pulp cavity. Nosmyth considers the fibres to bo rows of minute opaque 

 jM>ints arranged in a linear series (baccated fibres), and to be merely tho 

 nuclei of tho ivory cells, the interfibrous substance being tho remainder of 

 tho cell filled with calcareous matter.) 



Its chemical composition much resembles that of bones. After remaining 

 in dilute hydrochloric acid for several weeks, it comports itself like them, by 

 giving up the calcareous salts with which it is impregnated to the acid 

 solution, and becoming soft like cartilage ; submitted to tho action of boiling 

 water it yields gelatine. 



ENAMEL. The enamel extends in a layer over tho bony substance of the 

 free portion of the tooth, whose exterior it entirely envelops; it is prolonged 

 over tho fang in some animals, and in several kinds of teeth it dips by tho 

 crown into the interior of the organ to a very great depth. It is brilliantly 

 white, and so hard that it strikes fire like steel. 



. Its microscopic structure is very interesting, tho enamel being composed 

 of small prismatic hexagonal rods, l-5000th of an inch in diameter, and 

 notched on their faces. Owing to this notching, the prisms are intimately 

 united to each other. They form several layers which cross each other at 



Fig. 154. 



A. TRANSVERSE SECTION OF ENAMKI,, SIIO\V1\<; ll< II KXAGOXAI, PRISMS; 

 B. SEPARATED PRISMS. 



an acute angle, though in each layer they are exactly parallel to one another. 

 By immersing the enamel in dilute hydrochloric acid, there is detached 1'ioni 

 its surface a fine amorphous membrane or cuticle of Hie enamel. 



(The chemical composition of cuamel appears to be % 5 per cent, of 

 earthy matter, and 3'5 of animal substance. Tho first consi; 

 phosphate of lime, with traces of^ fluoride of calcium, carbonate of lime, 

 phosphate of magnesia, and other salts. The rods aro directed vertically 

 on the summit of the crown of the tooth, and horizontally at the sides.) 



CKMENT. (Gcim ntmii, eubstanlia oxto'ulcd, curtn-nl substance, or crusta 

 petrosa). The cement is spread in a non-continuous layer over the external 

 surface of the enamel and dentine. It is accumulated in largo quantity 

 in the substance of some teeth, as will bo noticed when speaking of tho 

 incisors in the Horse and the molars of the Herbivora. 



The structure and properties of this substance differ in nothing from 

 the structure and properties of tho spongy tissue of bono. In a physio- 

 logical state, tho cement does not contain any //</ />/'</// minis. (It 



