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134 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

" two factors have always been in operation
: (1) the action of
acids, and (2) the action of germs." " This theory—which for
the sake of distinction may be called the septic—is rather an
amplification of the chemical theory than a contradiction of it.
Most probably the work of decalcification is entirely performed
by the action of acids, but these acids are, we think, secreted by
the germs themselves, and the organic fibrils upon which the
organisms feed and in which they multiply are the scene of the
manufacture of their characteristic acids, which in turn decalcify
the matrix and discolor the whole mass." It need not be said
that the investio-ations of Milles and Underwood marked a n-nat
step in advance of anything hitherto accomplished in this direc-
tion.
Ad. WeiP" writes, "Decay generally begins from without, and
must, therefore, first make its way through the enamel-cuticle.
I regard it as highly probable that this fungus (Lepto-
.
thrix buccalis) bores directly through it. The fungi now pro-
ceed farther into the enamel and force apart its prisms, gradually
breaking up its structure. From the enamel they penetrate into
the tubules of the dentine, which they often enlarge to three
times their natural size, at the same time extracting the lime-
salts."
Arkovy ^'' describes caries as " a breach of continuity of the hard
dental substance brought about by chemical action, in which the
invasion of nosogenous fungi play an essential part."
Bastyr''^ : " As long as it cannot be shown that the appearances
observed in the decay of living teeth, decay of dead teeth in the
mouth, and artificial caries show any appreciable differences, so
long will every attempt to explain decay as a vital process be
very difficult."
Alfred Gysi^: " As all my experiments and investigations on
this subject have presented facts which are consistent with this
theory (the chemico-parasitic),! accept it as a satisfactory explana-
tion of dental caries."
Sudduth^'*' says, " Dr. Miller's theory of the formation of cavi-
ties by the action of a digestive ferment upon the basis-substance
I
of dentine has been the only theory ever advanced that explains
vthe formation of cavities."