Page 162 - My FlipBook

This is the reason we so often see those first molars in the
infirmary with the whole occlusal portion of the crown removed
by decay, instead of the deep, narrow cavity in the occlusal
surface that we find in teeth that are perfectly formed. Here
is a histological reason for this peculiar form of cavity in these
teeth. Now, there are very many imperfections that are of
lesser note than this, giving rise to deviations in the forms of
cavities that occur in the teeth, or deviations in the invasion of
micro-organisms. You will see, then, that in teeth of the very
best histological forms the penetration will be more directly of
the conical form,, and will, if beginning in the central area of
an occlusal surface, point directly toward the pulp of the tooth,
while in the imperfectly formed teeth it will be different.
The invasion of the dentinal tubules occurs little by. little^
purely by the spreading of the growth. The growth is slow
usually, consequently it requires a considerable time for the
invasion of the structure of the tooth. In some cases it goes
very much more rapidly than in others, depending, evidently,
upon conditions of the fluids of the mouth that favor or hinder
this growth. At first there will be just a few lines of micro-
organisms stretching along down a tubule, perhaps a single
line of micro-organisms ; then another line, and then another,
until the tubule will be filled full, as you see them depicted here
in this picture (indicating). Usually, in examining speci-
mens, we find that the invasion is very irregular; some of the
tubules will be filled full, while tubules lying beside them have
none, or very few, micro-organismis. They spread gradually
through the mass, the deeper portions having few and the por-
tions nearest the enamel being crowded very full. In cutting
a cross-section you will have something as shown in this chart
(indicating). Here are some tubules that have very few; here
are some that are very much crowded, and so on.
The lime salts are dissolved out of the dentin in advance
of the invasion of the micro-organisms; this is always the case.
We find it true in the examination of specimens that the soften-
ing precedes the invasion of the micro-organisms. First, we
have a plaque lying upon the enamel, and then the enamel
broken down and the tubules exposed to the growth of the
micro-organisms. But while this process is going on, the acids
being formed by the micro-organisms are percolating into the
tissue and destroying the lime salts in advance of the growing
150