of the nature of teeth is founded. It has been pointed out that the scales of the Elasmobranch fishes consist of a cap of enamel upon a base of dentine, the former being derived from the epidermis and modelled upon a papilla of the dermis whose cells secrete the dentine. The fact that similar structures arise within the mouth (i.e. the teeth) is explicable when it is remembered that the mouth itself is a late invagination from the outside of the body, and that therefore the retention by its tissues of the capacity to produce such structures is not remarkable.
Fig. 33.—Diagrammatic sections of various forms of teeth. I, Incisor or tusk of Elephant, with pulp cavity persistently open at base; II, Human incisor during development, with root imperfectly formed, and pulp cavity widely open at base; III, completely formed Human incisor, with pulp cavity opening by a contracted aperture at base of root; IV, Human molar with broad crown and two roots; V, molar of the Ox, with the enamel covering the crown deeply folded, and the depressions filled up with cement; the surface is worn by use, otherwise the enamel coating would be continuous at the top of the ridges. In all the figures the enamel is black, the pulp white; the dentine represented by horizontal lines, and the cement by dots. (After Flower and Lydekker.)
The relations of the three constituents of the tooth in its simplest form is shown in the accompanying diagram, where the intimate structure of the enamel, dentine, and cement (or crusta petrosa as it is sometimes called) is not indicated. The latter has the closest resemblance to bone. The dentine is traversed by fine canals which run parallel to each other and anastomose here and there. The enamel is formed of long prismatic fibres, and is excessively hard in structure, containing less animal matter than the other tooth tissues. To this fact is frequently
