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two modes of extension of micro-organisms into the dentinal
tubules that account for the forms of cavities of decay: First,
then, they grow directly into the tubules toward the pulp of
the tooth ; second, they spread laterally along the dento-enamel
junction, and as rapidly as a new tubule is reached they be-
gin to grow into that tubule, and as the next tubule is reached
they begin to grow along that one, etc., etc., and, continuing
that they form a cone. The first, in the center of the area, have
grown toward the dental pulp (indicating), and here these
that have entered somewhat to the side a little later also grow
directly toward the pulp, and those that have reached to a
greater distance along the dento-enamel junction also grow
directly toward the pulp (indicating), and so on indefinitely.
The result is that the first have reached the greatest depth,
those entering at the sides a less depth, so that we have a conical
invasion of the dentinal tubules. Now, this happens generally
in teeth that are well formed histologically.
There are a good many teeth that are not well formed;
—
there are a good many teeth that have interglobular spaces
spaces unfilled with lime salts. It is supposed that the dentin
is laid down in globules and the interspaces between the glob-
ules become calcified after they are laid down. In cases where
there has been illness that has interfered with nutrition during
the time the dentin has been forming, there will be portions
of dentin in which the interglobular spaces have not been filled
with lime salts completely, just as there are portions of enamel
that are not completely formed during these terms of illness
when nutrition has been interfered with ; and generally, where
you find the enamel is imperfect, you will find that the dentin
that was being formed at the same time is imperfect also.
Hence, if the enamel in a molar tooth has been illy formed on
account of an illness occurring during the time of its formation
there will be a line of interglobular spaces in the dentin cor-
responding in time of calcification with that fault in the enamel.
Now, just as soon as the micro-organism's in their invasion of
the dentinal tubules reach those interglobular spaces, they will
grow into those spaces and fill them full, and they being often

much larger than the dentinal tubules and giving easier in-
gress, the micro-organisms will begin to follow the line of those
interglobular spaces and follow it around through its whole
length, perhaps, spreading over the whole crown of the tooth.

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