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CHEMICAL CHANGES ATTESDING DECAY OF THE TEETH. 163

teeth of dogs to show a deep brown to black discoloration.
Most authors concur in the view that the pigment arises from
without, and is conditioned by causes which have nothing to do
with the decay itself.
"Watt explains it by the action of various mineral acids : mu-
riatic acid conditions the white, nitric acid the yellow, sulphuric
acid the brownish-black decay.
Clark "^ believes that the discoloration is called forth by the
color-forming power of bacteria.
Black ^'^ explains the discoloration by the settling of coloring-
matters into the parti}' decomposed tissue. These seem to be
derived chiefly from the dark sulphurets formed in the mouth
by the action of sulphuretted hydrogen upon such metallic ele-
ments as may be present. Others lay the blame on various foods
and stimulants, coffee, tobacco, etc.
It is not to be denied that smoking may occasion a dark dis-
coloration of dentine. It cannot, however, be the cause of pig-
mentation in dental caries, since the latter occurs in the cases of
persons who do not smoke. Then, again, dentine is colored
black also in the worn-ofl' teeth of dogs; moreover, discoloration
of dentine and coal-black deposits are very common in teeth of
animals without decay.
In my judgment, the cause of discoloration in dental caries is
exactly the same as that of the discoloration of any other organic
substance which is decomposed by micro-organisms. This idea
is presented at length in the chapter on chromogenic mouth-
bacteria.
CHEMICAL CHANCES ATTENDING DECAY OF THE TEETH.

With very few exceptions, all investigators Avho have given any
attention to the study of the phenomena of dental decay have
been unanimous in the opinion that the softening of the dentine
is caused by the extraction of the lime-salts.
The fact that we have to do with a genuine process of decal-
cification is so patent that I would have no excuse for presenting
the experiments recorded below, if they were not of some value
in showing the comparative degrees of decalcification in decayed
dentine and in dentine artificially softened, as well as the com-