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OF THE STRUCTURE OF A TOOTH, ETC. 43

Teeth, (o) On the base or grinding surface it is of a pretty equal
thickness, and therefore is of the same form with the bony sub-
stance which it covers.
It would seem to be an earth united with a portion of animal
substance, as it is not reducible to quick lime by fire, till it has
first been dissolved in an acid. When a Tooth is put into a
weak acid, the Enamel, to appearance, is not hurt; but on
touching it with the fingers, it crumbles down into a white pulp.
The Enamel of Teeth, exposed to any degree of heat, does not
turn to lime : it contains animal mucilaginous matter ; for when
exposed to the fire, it becomes very brittle, cracks, grows black,
and separates from the inclosed bony part of the Tooth, (p) It is
capable, however, of bearing a greater degree of heat than the
bony part, without becoming brittle and black.* This substance
* From this circumstance we can shew the Enamel better by burning
a Tooth, as the bony part becomes black sooner than the Enamel. The
method of burning, and shewing them after they are burnt, is as
follows.—Let one half of a Tooth be filed away, from one end to the
other, then burn it gently in the fire ; after this is done, wash the filed
surface with an acid, or scrape it with a knife. By this method you
will clean the edge of the Enamel, which will remain white, and the
bony part will be found black.
course. On the contrary, they are everywhere slightly and irregularly
waved. On the summit of the crown they present two or three larger
waves. Decussations of layers of the enamel prisms take place in transverse
planes of the tooth. Hence it is that, on the addition of hydrochloric
iicid, a longitudinal section will have a striated appearance, the prisms
cut transversely will appear darker than those which present their
longitudinal aspect. Layers of the enamel fibres may be seen on the
summit of the crown to run in an annular form, describing circles on
the molar, ellipses on the cutting teeth. Besides the striated appearance
indicating the stratified arrangement of the fibres, Kolliker describes
certain brownish or colourless lines crossing the enamel fibres in
different directions, which in perpendicular sections are seen as obliquely
ascending lines or arches, but in transverse sections as circles in the
outer layers of the tissue, rarely occurring throughout its entire sub-
stance. He regards them as " the expression of the lamellated mode of
formation of the enamel."] (1)
(o) [The enamel extends lower upon the inner and outer than upon
the opposed surfaces of the crowns.]
Q)) [Enamel in chemical composition differs from the other tissues of
(1) Kolliker, op. cit., pp. 296, 297.