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lungs, and in various parts of the body we will find it in
association with other micro-organisms within the tissue.
But growing in the mouth it seems to be a harmless organ-
ism. It is a peculiar fact that most of the micro-organisms
that grow habitually in the mouth become attenuated in
that position and are not as likely to do damage after they
have grown for some generations as they would if taken
from some other source.
There is another form that I have not an illustration of.
That divides into eight, known as packet fungi. The sar-
cina, that we find in the human stomach frequently, is one
of the best examples of it. You will see that the tetrogen
divides on two dimensions, while the sarcina divides upon
three dimensions, forming packets.
(Changing slides. Fig. 8.) This is a micro-organism that
we find frequently in the human mouth. It is not a good
grower and is not seen so often as some of the other forms.
It was described first by Miller, but he did not give it a
name, and I have dropped into the habit of calling it coccus
irregularis, because it is irregular in every feature of its
growth. Now, these small cells are just as complete as the
large ones, and they are of the same species. Here are
some very large ; they divide irregularly ; they are irregular
' 7/
!j' in their size ; they are irregular in every way. Here is one
that seems to be dividing in three. And so you will find all
sizes mixed up together. One may plant from colony after
colony in the effort to divide the one from the other, and you
will find every new colony presents all these varieties of
form. It is one species only, notwithstanding the great varia-
tion of size and form and method of division. This micro-
organism forms acid freely, and I see no reason why it should
not be a caries fungus, and yet I have never found it in the
deep layers of carious dentin. We will often find it in the
outer layers of carious dentin.
(Changing slides. Fig. i.) I want to say one word more
about the growth of this organism, the staphylococcus.
While it divides in twos, it is irregular as to the pole. You
see this zigzag chain here. It forms that zigzag by the irregu-
larity of the pole of division, and for that reason it forms a
bunch, not in regular streptococcus ^orms, but this peculiar
irregularity of its division causes it to form into bunches

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