Page 151 - My FlipBook

there is a little difference in the refraction of light by the
cementing- substance and the rods, enabling you to see the
rods in good specimens with the microscope. If you will
grind a specimen of enamel and cement one side to your glass
slide with balsam, you may lay the acid upon the upper side
and let it act for a few moments, until you see just a little ol
bubbling begin to rise from it, then wash it off and examine
it with* the microscope, and you will find that the rods stand
out so that you will see them very much better, because this
cementing substance has been dissolved slightly from between
the rods—the etching process, as we call it in the preparation
of these specimens. In these several ways you will learn
how the enamel is dissected and taken apart by the action of
an acid. And this process is the process of tlie beginning of
caries in the enamel as we find it. If the process of solution
has gone a little farther we will find that the enamel has be-
come friable ; it may not be all gone, may not be broken up,
but has become friable so that we may press a sharp ex-
plorer into it and break up the rods, perhaps the wdiole dis-
tance through, for it often happens in the beginning of caries
in the enamel that the acid is absorbed into its substance to
a considerable depth before the enamel is broken up. Occa-
sionally we will see these spots wdiere the enamel seems quite
perfect upon the surface—it has lost its luster, it is opacjue,
white, perhaps—where we can break up the whole thickness
of the enamel easily with an excavator, or even with a sharp
explorer, possibly it will break up into a white powdery sub-
stance very readily, and, taking this to the microscope, we
will find that it is composed of fragments of enamel rods.
I have tried to illustrate this here (referring to chart).
This little color you see here represents a plaque of gela-
tinous substance formed by micro-organisms upon the sur-
face of the toothy and this darker portion is where the enamel
is affected by the acid, percolating clear through its thick-
ness, and even into the dentin, before the enamel is broken
up. In grinding sections we sometimes run across this kind
of an eff'ect of the acid upon the enamel, and even upon the
dentin, before the enamel is apparently broken up. So that
the effect is often very deep in the enamel before the surface
is broken. In other cases the effect seems to be confined
more closely to the outer surface of the enamel. We find all
grades of this effect, from very slight to considerable perco-