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170 HUNTER ON THE TEETH.
It does not arise from any external injury, or from menstrua,
which have a power of dissolving part of a tooth for any-
;
tiring of that kind could not act so partially ; and we can
observe in those Teeth where the disease has not gone deep,
that from the black speck externally there is a gradual decay

composed by the fluids of the mouth. That there must be a concur-
rence of dead dental tissue, and of a condition of the oral fluids capable
of decomposing the dead part before the phenomena of caries can be
developed.
" Further, I conceive that the causes producing the abnormal action
may have been applied locally to the tooth itself, or may have had a
constitutional origin, and therefore have acted through the nerves or
the circulating fluids."
The latter theory appears to afford the most correct explanation of the
nature of dental caries. At the same time there is scarcely sufficient
proof that the part is actually dead before caries can commence, and it
would perhaps be more correct to say that the vitality of the part is so
far diminished or altered as to render it unable to resist the chemical
action of external agents. One point of some importance seems to have
been overlooked by most writers on caries. Great attention has been
paid to the condition of the saliva, but thu state of the pulp and of the
nutrient fluid which it conveys into the dentine has been altogether
omitted. It is well known that the serum of the blood is alkaline, and
Professor Wurtz found, upon analysing the pulp of the tooth, that this
also was strongly alkaline ; hence it is only reasonable to conclude that
the fluid pervading the dentine is the same. The origin of caries may
therefore depend upon the relative state of the alkaline and acid condi-
tions of the pidp and of the saliva, not only may it arise from excessive
acidity of the saliva, but it may also depend upon a want of alkali in
the pulp and fluids of the tooth.
While nearly every recent authority attributes caries to the action of
an acid, it is still a question what is the nature of this acid, and how it
is generated. Harris mentions nitrous acid, but I know not upon what
authority, nor am I aware of any proof that decomposing food gives rise
to this acid. In a paper read at one of the meetings of the Odontologi-
cal Society, Mr. Bridgeman endeavoured to show that the destructive
action was produced by lactic acid, founding his views upon the state-
ment that moist animal membrane in a slightly decaying condition
often acts energetically in developing lactic acid. (1)]

(1) See British Journal of Dental Science. Vol. 4.