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180 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

382 ""^ is correct in every respect : " The action of acids always
precedes the invasion of bacteria."
As is best seen in a longitudinal section under low power,
numerous rounded or oval masses of from ten to one hundred
micro-millimeters long and from five to fifty broad often appear
in the domain of the infected dentine (Fig. 74). The masses con-
sist of closely packed cells of bacteria, and correspond to the
more or less ample enlargement of one or more dentinal tubules,
bv which the neischborine; tubules are crowded together or bent
out of their course. If these masses attain larger dimensions, the
coarse of some of the tubules, together with the intervening basis-
substance, is interrupted for a certain distance, thus forming a
space or cavern in the dentine. These caverns being formed by
the dissolving of the basis-substance by peptonizing bacteria,
may be properly designated as liquefaction-foci.
Such foci do not al-
FiG. 75. Fto. 7fi. ^vays have the form
represented in Fig.
74 ; they may be
tapering, triangular,
crevice-shaped, etc.
The crevice-shaped
s])aces frequently lie
obliquely to the den-
hi J,'
'k tubules, pre-
tinal
senting the charac-
teristic appearance
Uniform En- shown in Fig. 75.
largement OP
i\' In some specimens
the Tubules
IN Decaying all the tubules are
Dentine.
V
enlarged to nearly the
Decayed Dentine with Oblique
same extent (Fig. 76),
Liquefaction-Foci. Circa 4(30 : 1.
occasionally to three
or four times their normal size. This enlargement is caused
partly by the dislodgment of the neighboring tubules,partly by the
loss of the intertubular substance. Lastly, the destruction of the
intervening substance causes a confluence of two or more adjacent
tui)ules, producing long caverns running parallel to them. By the