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MICROSCOPICAL PHENOMENA OF DECAY. 197

or depressions, the whole tissue had been converted into a soft
mass such as is often found on the surface of third mohirs.
In two cases, apparently on account of a defect of structure,
the cusps of the teeth had been attacked. On thoso points
where the acid had penetrated through the enamel, its action
upon the dentine could be followed in all directions; where the
enamel was hard and thick, without cracks or other defects, it
had not even lost its natural polish.
All of the phenomena observed in cases of so-called white
decay were present in these cases of artificial decay. If the
mixture was allowed to stand until tlie reaction became alkaline,
or if the pieces were exposed to tlie air or to the action of dif-
ferent articles of food, such as coffee, tea, tobacco, fruit, etc., all
possible shades of color were produced just as they are found in
the mouth.
These experiments show, among other things, to what an
extent the resistance which the tooth opposes to the destroying
factors depends upon the structure; further, it furnishes an
answer to the question why all teeth under the same conditions
d» not become decayed in the same degree. A tooth of sound
structure, protected by sound enamel, will resist the action of an
acid many years, whereas a soft, imperfectly developed tooth
under the same conditions would show decay in the course of a
few weeks.
Sections of these pieces showed all the microscopic changes
which have been described as characteristic of decay of dentine.
(See Figs. 99 to 101.) The canaliculi were tilled with bacteria,
and at many points were much distended; tlie thickening of
Neumann's sheaths and the swelling of the fibrils could also be
well observed. A well-known dentist and histologist, to whom
I showed one of the preparations, at once called my attention to
the varicose swelling of the fibrils and to the distended canaliculi,
not knowing that he had an artificial preparation before him.
Up to the present, not one who has made the attempt has been
able to distinguish a specimen of artificial from one of natural
decay at its side.
AVhen Atkinson ^^ says, " There is not one of these cases that
cannot be discovered in an instant as to which is natural and