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154 THE MICRO ORGANISMS OF THE HUMAN MOUTH.

an! porous, infiltrate 1 and contaminated with particles of fer-
menting food, masses of bacteria, etc.
The discoloration of the dentine is brought about bj the same
causes as that of the enamel ; it also shows the same variations in
acute and chronic decay, and all shades, from the natural color of
dentine to black.
As the destructive process spreads more rapidly in dentine than
in enamel, the latter becomes usually more or less undermined,
and the cavity may acquire a shape resembling a short-necked
The undermined
Florence ilask (Fig. 58).
Fig. 58. enamel-margins become dry and brittle, and
easily break oft'; it therefore not unfrequently
happens, particularly in case of extensive
decay on the approximal surface, that the
cover of enamel breaks under the pressure
exerted by mastication and reveals a large
UxDERMixixG Decay
Flask-shaped cavity. previously invisible cavity. Or, decay pro-
ceeding from the grinding-surface of molars
destroys the greater part of the dentine, so that only a perforated
enamel-cap remains; this finally breaks into pieces at the neck
of the tooth, thereby completing the de-
Fig. f.9.
struction of the crown. In rare cases,
caries beginning on the grinding-surface
seems to proceed particularly rapidly at
the border between enamel and dentine.
The bond of union between the two tis-
sues is weakened or destroyed, the enamel-
walls break away, while a large portion of
the decalcified dentine remains (Fig. 59).
In other cases, again, the destructive
process advances most rapidly along the
line of the dentinal tubules toward the
pulp. By this means a tube-shaped cavity
Decay oi- a Milk-Molar.
is formed, as is often observed on the
Tho en;iinel-wal]? are broken
off, while the dentine re-
grinding-surface. These cases are usually
mains standing.
designated as penetrating decay. But
these terms, penetrating and undermining decay, must not mis-
lead us, for there is no specific form of decay which shows a