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SIXTEENTH LECTURE.

The sixteenth lecture consisted of a review of
the previous lectures on caries of the teeth. The va-
rious processes were illustrated by photo-micrographs
in the form of lantern slides projected on the screen.
Forty-four were used. First the process of decay
of the enamel was illustrated, showing the microbic
plaque formed on the surface of the enamel, the
percolation of the acid formed into the substance of
the enamel, and the beginning of the solution of the cement-
ing substance between the rods. Then the further progress
by the breaking up and final solution of the rods them-
selves, moving progressively into, and through, the enamel,
destroying it entirely. In the photo-micrographs shown,
some showed very deep percolation of acid into the tissue
of the enamel before it began to break up, and others showed
the process more complete near the surface with less depth
of penetration, giving the idea of much difference in the
process of penetration.
Next followed a series of views illustrating the process
of caries of the dentin, showing the invasion o{ micro-or-
ganisms into the dentinal tubules, the hyaline zone of Tomes,
and the conical forms of the areas invaded. This conical
form oi the area of decay was shown to be caused by the
gradual spreading of the micro-organisms along the dento-
enamel junction and growing into the tubules as they came
to them, so that those that started in the central part of
the area were always in advance of those in its lateral parts.
This created the conical form. In some of the cases illus-
trated the cone of invasion was deep and narrow, i. e.,
the penetration was considerable toward the pulp of the
tooth while the spreading along the dento-enamel junction
was slight, thus forming a deep, narrow area of decay.

In other cases illustrated the invasion extended more rap-
idly along the dento-enamel junction than toward the pulp

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