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ETIOLOGY OF DENTAL DECAY. 211

pockets or spaces which, by retaining food-particles, serve as
centers of fermentation and consequent decay.
According to Coleman,^^ the acid reaction of decayed dentine
is caused by the formation of an acid phosphate of lime, arising
from the disintegration of the lime-salts.
Bridgman ^-* explains the aciditication by an electrolytic de-
composition of the buccal juices.

^^
. The second stage of caries^ihQ dissolution of the softened dentine,
is caused by bacteria. We have seen that many mouth-bacteria
have the faculty of dissolving coagulated albumen or albuminous
substances, of peptonizing or converting them into a soluble
modification. AVe have also seen that the basis-substance of
dentine consists of an albuminous substance. The explanation 7
of the second stage of decay is therefore very easy, the more so, >
since the liquefaction of the softened dentine by bacteria is i
directly detectalile under the microscope and may be easily ac- ^
complished experimentally. The dissolution of dental cartilage
(in fact, decay in general) has been designated as putrefaction, on
the whole an ill-chosen term, inasmuch as the characteristics
of putrefaction (alkaline reaction and bad odor) are entirely
wanting in a cavity of real decay. The decaying dentine shows
an acid reaction and emits a sour smell.* This stage of caries,,
therefore, is a digestive process. The dental cartilage is dissolved
by the bacterium-ferment, as albumen by the pepsine of the
gastric juice.
We must rid ourselves of the impression, which the applica-
tion of the very unscientific and unprofessional name of bugs
to bacteria has no doubt tended to spread, that the parasites
of the human mouth make holes in the dentine by boring
into it as a worm bores into wood or by gnawing at it as a dog
gnaws at a bone. Bacteria have no apparatus for boring, nor do
they have mouths or any provision for breaking oflJ" small por-
tions of solid substances which they then swallow whole or take
directly up at any point of their periphery after the manner of an
amceba. They nourish themselves alone by substances in a state

*0f course the odor of a gangrenous pulp or of suppurating gums, etc., must
not be confounded with that of the dentine.