a. The enamel.
The enamel consists, according to Purkinje, of square, or, according to Retzius, of hexagonal closely-aggregated prisms, which stand nearly perpendicular upon the surface of the ivory, and pass outwards in a slightly curved direction. It is at first soft, and if some of it be scratched off in that state, we obtain, what Miller has described as needle-shaped bodies pointed at both extremities. According to Purkinje, Raschkow, and Retzius, some organic substance remains after the young enamel has been treated with hydrochloric acid, whilst Berzelius asserts that the enamel of mature teeth does not contain two percent of organic matter. For further details I refer the reader to the excellent works of Purkinje, Raschkow and Fränkel, and those of Retzius, J. Müller, and v. Linderer
If an immature tooth of a child or mammal (the pig, for instance) be removed from its capsule and placed in dilute hydrochloric acid, the organic substance of the enamel which remains after the solution of the earthy matter, may be sepa- rated from the ivory entire. It has exactly the form and size of the enamel previous to the action of the acid. It is very soft, and breaks readily in the direction of the fibres of the enamel. Examined with a high magnifying power and subdued light, it is found to be composed, like the enamel itself, of closely-aggregated prisms, which may be insulated from one another, so that each one forms an independent structure. (See pl. III, fig.3.) This organic substance, therefore, cannot be, as Raschkow and Retzius considered, a mere deposit from the moisture with which the enamel-fibres are at first surrounded, and thus a sort of cast of the enamel-fibres, but either the fibres must result from an ossification of these; prisms, or the prisms must be hollow, and the inorganic substance deposited within them. When the enamel of the pig’s tooth is examined with a subdued light, the contour of these organic prisms is found to be so dark in comparison with their interior, that it can scarcely be regarded as the mere | shaded outline of a solid prism, but suggests the idea of a cavity surrounded by a thin membrane. ‘This distinction is,
