98 ENAMEL OF THE TEETH. 
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 Miiller 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 per 
cent. of organic matter. For further details I refer the reader 
to the excellent works of Purkinje, Raschkow and Frankel, 
and those of Retzius, J. Miller, 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 sur- 
rounded, 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 imorganic 
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, 
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