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ENAMEL. 99

found in the fully formed tissue being the result of imperfect execution
of the plan.
The enamel is coni])()sed of two structural elements, the enamel rods,
or prisms, sometimes called enamel fibers, and the interpeismatic or
CEMENTING SUBSTANCE, both of which are calcified. It is to the arrange-
ment of these structural elements that the characteristics of the tissue
with \\hich we are most concerned in operative procedures are due.
While both the prisms and interprismatic substance of the enamel
are calcified, or, better, composed of inorganic salts, the two substances
—that is, the substance of the rods and the substance between the rods
—show markedly different properties both chemical and physical. If
treated with acid, the interprismatic substance is acted upon more
rapidly than the rods, so that the latter become more conspicuous. By
this means sections of the enamel may be etched to render it easier to
study the direction and arrangement of the rods. If the action of the
acid is carried far enough, the rods will fall apart before they are them-
FiG. 79.

Enamol roils isolatL'd Ijv caries. (About li

selves entirely dissolved. Fig. 79 is from the debris in a carious cavity,
and show^s rods isolated by the action of the acids of caries.
The interprismatic substance is not as strong as the rods, so that in
splitting or breaking the enamel the tissue separates on the lines of the
cementing sul)stance, occasionally breaking across a few rods but fol-
lowing their general direction, the lines running between rods, not at
their centres.
In cleaving the enamel the eiiisel does not enter the tissue sepa-
rating rod from rod, but the edge engages with the surface, and the