S46 



THE DIGESTIVE APFABATUS IN MAMMALIA. 



its mass are deposited the saline molecules which give the dentine its bony 

 consistency. (These molecules are deposited in lamellse, concentric with 

 the pulp cavity. Nasniyth considers the fibres to be rows of minute opaque 

 points arranged in a linear series (baccated fibres), and to be merely the 

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

 the cell filled with calcareous matter.) 



Its chemical composition mucb 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 wbicb it is impregnated to the acid 

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

 water it yields gelatine. 



Enamel. — The enamel extends in a layer over the bony substance of the 

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

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

 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, the 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. TEANSVEESE SECTION OF ENAMEL, SHOWING ITS HEXAGONAL PRISMS; 

 B. SEPARATED PKISMS. 



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

 By immersing the enamel in dilute hydrochloric acid, there is detached from 

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



(The chemical composition of enamel appears to be 96-5 per cent, of 

 earthy matter, and 3-5 of animal substance. The first consists of 

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

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

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



Cement. — (Cementum, substantia ostoidea, cortical substance, or crusta 

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

 sm-face of the enamel and dentine. It is accumulated in large quantity 

 in the substance of some teeth, as will be noticed when spealdng of the 

 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 the spongy tissue of bone. In a physio- 

 logical state, the cement does not contain any Haversian canals. (It 



