DENTAL SYSTEM 15 



matrix, something like, but not identical with, that of bone, richly 

 impregnated with calcareous salts (chiefly calcium phosphate), these 

 constituting in a fresh human tooth 72 per cent of its weight. 

 When subjected to microscopical examination it is seen to be every- 

 where permeated by nearly parallel branching tubes which run, 

 in a slightly curving or wavy manner, in a general direction from 

 the centre towards the free surface of the tooth. These tubes com- 

 municate by open mouths with the pulp-cavity, and usually ter- 

 minate near the periphery of the dentine by closed ends or loops, 

 though in Marsupials and certain other mammals they penetrate 

 into the enamel. They are occupied in the living tooth by soft 

 gelatinous fibrils connected with the cells of the pulp. A variety 

 of dentine, permeated by canals containing blood-vessels, met with 

 commonly in fishes and in some few mammals, as the Megatherium, is 

 called vaso-dentine. Other modifications of this tissue occasionally 

 met with are called osteo-dentine and secondary dentine, — the 

 latter being a dentine of irregular structure which often fills up the 

 pulp-cavity of old animals. 



The enamel constitutes a thin investing layer, complete or 

 partial, of the outer or exposed and working surface of the dentine 

 of the crown of the teeth of most mammals. This is the hardest 

 tissue met with in the animal body, containing from 95 to 97 per 

 cent of mineral substances (chiefly calcium phosphate and some 

 carbonate, with traces of fluoride). Its ultimate structure consists 

 of prismatic fibres, placed generally with their long axes at right 

 angles to the free surface of the tooth. Enamel is easily distin- 

 guished from dentine with the naked eye by its clear, bluish-white, 

 translucent appearance. 



The cement or crusta petrosa is always the most externally placed 

 of the hard tissues of which teeth are composed, as will be under- 

 stood when the mode of development of these organs is considered. 

 It is often only found as a thin layer upon the surface of the root ; 

 but sometimes, as in the complex-crowned molar teeth of the Horse 

 and Elephant, it is a structure which plays a very important part, 

 covering and filling in the interstices between the folds of the 

 enamel. In appearance, histological structure, and chemical com- 

 position it is closely allied to osseous tissue, containing lacuna? and 

 canaliculi, though only when it is of considerable thickness are 

 Haversian canals present in it. 



Development. — The two principal constituents of the teeth, the 

 dentine and the enamel, are developed from the two layers of the 

 mucous membrane of the jaw — the dentine from the deeper or vas- 

 cular, the enamel from the superficial or epithelial layer. The latter 

 dips down into the substance of the gum, and forms the enamel-organ 

 or germ, the first rudiment of the future tooth, which is constantly 

 present even in those animals in which the enamel is not found as a 



