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60 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

happens, however, that bacteria which we have obtained on agar-
agar do not grow at all on gelatine, because of the low tempera-
ture at which it must be kept, just as it may also happen that
cultures made from the saliva on gelatine may develop nothing
whatever, although the saliva at the time may have contained
bacteria enough. If, therefore, I wish to obtain cultures from the
deposits or accumulations, secretions, etc., of the human mouth,
I make line cultures on agar-agar, either alone or supplemented
b}' dilution cultures on gelatine. For particular purposes, as in
the attempt to cultivate the Spirillum sputigenum, etc., I would
of course employ various other materials.
Pure cultures of the bacteria of tooth-decaij I obtain by the follow-
ing method, which varies somewhat, according to whether we
wish to secure cultures from the surface or from the deeper part
of the dentine : In the former case, after having removed the
remains of food from the cavity of decay, I scratch out a small
portion of the decomposing dentine with a sterilized excavator;
this is then transferred to a plate of agar-agar in the manner
above described. To obtain a pure culture from the deeper
parts of the dentine, I first wash the tooth in a stream of pure
water and place it for some minutes in a 5 per cent, solution
of carbolic acid, also brushing carefully with the same solution,
so as perfectly to remove all traces of food and to destroy those
bacteria which are found only on the surftice. I then dry the tooth
with sterilized bibulous paper, and remove the outer layers of
decayed dentine with a sterilized, spoon-shaped excavator. With
a second excavator I remove a second layer, with a third a third
layer, and so on, till I arrive near to the border of the sound
dentine; or, I undercut the decayed dentine at one side of the
cavity with a sharp instrument, and, grasping the detached edge
with a strong pair of pliers, I shell out the largest part of the
deca^'ed matter in one piece. In either case I then detach a
small portion of the decayed dentine from the bottom of the
cavity, and place it in a drop of sterilized water, where it is torn
to pieces with two stiff needles, or fine excavators. With these
pieces, and with the water, the agar-agar plate is then inoculated.
At the same time dilution cultures may be made upon plates
of nutritive gelatine. These must of course be kept at a lower