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MICROSCOPICAL PHENOMENA OF DECAY. 181

formation of caverns and by the fusion of adjacent caverns the
dentine is broken up, becomes porous, and is gradually de-
stroyed. The tubules are often attacked in groups, while others
lying between these groups may be wholly free from infection. In
other cases, however, every single canal is stuifed full of bacteria.
In Fig. 77 I have endeavored to reproduce an appearance
very commonly met with in decaying dentine. This specimen

Fig. 77. Fig 78.

W

•St.!

A SiNCiLE
Tubule from
the preiiaration
illustrated in
Fig. 77.
clearly shows that the enlargement
of the tubules always found in
Decayed Dentine showing Total decayed dentine is due to the in-
Liquefaction oe the Basis-Sub-
fection, and that a gradual ftision
stance BY Bacteria. 400 : 1.
of the basis-substance may take
place, beginning with the separate tubules, until nothing remains
but a mass of bacteria held together by the remnant of the den-
tine. Fig. 78 shows a tubule fi'om the same specimen under
high power.
In rare cases the action of the bacteria is mostly restricted to
the surface, and the dentine is gradually dissolved from the sur-
face downward, without the formation of caverns within the
dentine. I term this form of dissolution of dentine progressive