Page 240 - My FlipBook

212 THE MICRO-ORGANISMS OF THE HUMAN MOUTH.

of solution, and if we present tliem solid substances they them-
selves must liquefy them before they can make any use of them
for their own nourishment.
y Upon this power of bacteria to liquefy substances of an albu-
minous nature depends the destruction of the softened dentine,
—in other words, the second stage of dental decay.
The olnjection has l)een raised against the chemico-parasitical
theory of caries, that the reaction of the saliva in any mouth is
no criterion for the extent of decay in that mouth. AVhen the
reaction of the saliva is alkaline, decay has heen found to be
extensive ; and, on the other hand, cases have been reported
where an acid reaction of the saliva was not accompanied by a
corresponding amount of decay. Xone but a very superficial
investigator, however, would draw conclusions from a simple
examination of sahva.
The rapidity with which the process of destruction of the
teeth in any mouth advances is evidently dircctl}' })roportional
to the intensity of the fermentations going on in the retention-
centers, and inversely proportional to the density of the tooth-
substance. Now, both of these factors are virtually independent
of the reaction which the saliva may show on escaping from the
ducts. A prolonged strong acid reaction of the saliva would
indeed render the fluids of the mouth less adapted to the devel-
opment of bacteria, and in so far as the acid could penetrate the
centers of fermentation tend to decrease the intensity of such
fermentation. Such a decrease would, however, be compensated
for by the action of the acid itself.
The case becomes very different when we turn our attention
to the fi'ee surfaces of the teeth. A prolonged acid reaction of
the saliva would of necessity manifest its disastrous influences
upon these surfaces sooner or later, according as the teeth are
soft or dense in structure. Consequently, if anyone can shd^\"
me a case in which an acid condition of the saliva has persisted'
for some months (not one in which tbe saliva chanced to react
aci'd just at the moment of examination, or one in which the acid
reaction was caused hx l)ad litmus-paper, or by handling the
paper with sweaty Angers, etc.), I shall have no difficulty in
pointing out places at which its action is jilainly manifest.