into apparently fine prismatic fibres; if these fibres be separately
treated with dilute muriatic acid and the residue examined, with a
moderate magnifying power, in distilled water, or, better, in dilute
alcohol, portions of more or less perfect membranous sheaths or tubes
will be discerned, which inclosed the earthy matter of the minute
prism, and served as the mould in which it was deposited.
Professor Retzius, who obtained a small portion of organic or
animal substance from the enamel-fibres of an incompletely-formed
tooth of a horse, conjectured that it was a deposition of that fluid
which originally surrounds the loose enamel-fibres, and that, “ in
proportion as these fibres are pressed tighter together, and additional
fibres are wedged between them, the organic deposition is forced
away.”
It is certain that the small proportion of animal matter which can
be obtained from the enamel of a tooth, that has been completely
formed and in use, does not yield any indication of its primitive organic
form; this may, however, be ascertained, if the enamel be
examined under the conditions above described. The tubular structure
of the membranous constituent of recently formed enamel has
been observed by Dr. Scbwann(l) in the teeth of the hog: and he
has shown that the fine membrane of the enamel-prism is not a mere
deposition from the fluid in which the new-formed prisms are bathed,
but an organized part specially formed and arranged in the enamel
pulp in order to ensure the right disposition and direction of the calcareous
salts of the enamel.
Retzius accurately describes the enamel-fibres of the horse as
presenting the form of angular needles, about ^ th of an inch in
diameter, which are traversed by minute and close-set transverse
(1) Loc. cit. p. 118.
striae, over the whole, or a part of the fibre ; and he conjectures that
if the enamel-fibre be a mass of the calcareous salts, surrounded by
an organic capsule, that the striae may then belong to the capsule and
not to the enamel-fibre. The later researches of Dr. Schwann add to
the probability of this conjecture, and the absence of the minute striae
in the enamel of fossil mammalian teeth, at least in the examples
which I have submitted to microscopic investigation, may depend
upon the destruction of the original organic constituent of the
enamel.
The enamel-fibres are directed at nearly right angles to the surface
of the dentine, and their central or inner extremities rest in slight
but regular depressions on the periphery of the coronal dentine. Thus
in the human tooth, the fibres which constitute the masticating surface
are perpendicular or nearly so to that surface, while those at the lower
part of the crown are transverse, and consequently have a position best
adapted for resisting the pressure of the contiguous teeth, and for
meeting the direction in which external forces are most likely to impinge
upon the exposed crown of the tooth. The strength of the enamel
fibres is further increased by the graceful wavy curves in which they
are disposed; these curves are in some places parallel, in others opposed;
their concavities are commonly turned towards each other where the
shorter fibres, which do not reach the exterior of the enamel, abut by
their gradually attenuated peripheral extremities upon the longer fibres.
Other shorter enamel-fibres extend from the outer surface of the enamel
towards the dentine and are wedged into the interspaces of the longer
fibres. In the teeth of fishes, the calcigerous tubes or fibres of
the enamel, which ramify and subdivide like those of the dentine,
have their trunks turned in the opposite direction, or towards the
periphery of the tooth ; so likewise even in the human teeth the analo