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PHYSICAL PHEXOMESA OF DENTAL DECAY. 153

b. Decay of Dentine,
By the progress of the above-described process the destruction
of the enamel spreads until the surface of the dentine is reached
(Fig. 57). Here the disease takjs on a very ditierent form, inas-
much as we no longer find
the tissue being changed into Fig. 57.
a soft cheesy mass, but into a
tough cartilaginous substance
which does not readih- fall to
[)ieces or yield to the slightest
friction, as docs the decayed
enamel, but may retain its
form for some time. This
stage is designated as soften-
ing of dentine, and is condi-
tioned, as will be seen below,
by a more or less complete
decalcification of the dentine.
The softening spreads in all
directions in the dentine, with
a rapidity dependent upon the
intensity of the fermenta-
tion processes present in the
mouth and the physical and
chemical constitution of the
dentine.
The softened mass may be
easily cut or peeled otf with
USDERMINIXG EXAMEL DeCAV.
a sharp instrument; upon a, Massas of bacteria lining the cavity. Cimi5):l.
pressure, it discharges a small
quantity of a liquid which in the great majority of cases will be
found to redden blue litmus-paper, /.e., to have an acid reaction.
The thickness of the .softened la\'er varies considerably in
difterent cases, for reasons which will be fully discussed in
Chapter Xlll. Very soon after the softening of the dentine its
disintegration or dissolution begins, leading to the formation of
;
a eavitj' in the dentins. Thj surface now appears uneven, soft,y