THE MOUTH. 



413 



the enamel ; single at their origin, they soon bifurcate, and again anastomose 

 several times during their slightly undulating course. They terminate in a 

 cul-de-sac, or in irregular cavities situated beneath the enamel, and named the 

 interglobular spaces of Gzermak (forming the interglobular or nodular layer). 

 These canals have a thin proper wall, and contain a dental fibre, which very 

 probably is a continuation of the pulp-cells. The fundamental substance (or 

 matrix) is amorphous, and not very abundant ; in its mass are deposited the 

 saline molecules which give the dentine its bony consistency. (These molecules 

 are deposited in lamellas, concentric with the pulp cavity. Nasmyth 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 much resembles that of bones. After remaining 



Fig. 226. 



SECTION THROUGH THE FANG OF A MOLAR TOOTH. 



A, a, Dentine traversed by its tubuli ; 6, 6, interglobular, or nodular layer ; c, c, cementum. 



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 the action of boiling water, it 

 yields gelatine. The mineral matters of dentine differ from those of bone in 

 having a smaller proportion of carbonate of lime. 



Enamel. — The enamel extends in a layer over the ivory of the free portion 

 of the tooth, the exterior of which 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, s-^oo 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 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 from its surface a fine 

 amorphous membrane {cuticle of the enamel). 



(The chemical composition of enamel appears to be 9 6 '5 per cent, of earthy 

 matter, and 3"5 of animal substance. The first consists of phosphate of lime, 

 with traces — 3 per cent. — of fluoride of calcium, carbonate of lime, phosphate 

 29 



