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CARIES OF ANIMAL TEETH. 199

natural decay the micro-organisms in the deeper parts of the de-
caj-ing tissue are confined strictly to the tubules, whereas those
nearer the surface, although they do not strike into the basis-sub-
stance, yet they gradually liquefy it, and thus produce caverns or
microscopic holes in it which they immediately fill up.
My experiments have been repeated and their results con-
firmed by Foerster. Since 1884 I myself have repeated the
experiments a number of times, and have somewhat changed the
conditions by adding meat to the mixture and changing it every
second or third day. Xot unfrequently the course of the ex-
periment is interfered with by the appearance of yeast-fungi,
particularly of Saccharomyces mycoderma, iu the mixture. Tliis
fungus appears as a white, thick, dry, felty skin upon the suriace
of the mixture, and uses up the acid. In the course of a few days
putrefaction sets in and the mixture shows an alkaline reaction,
by which the course of the experiment is interfered with. If no
such disturbance occurs, the pieces will be so far decalcified in a
w^eek that they may be easily taken up with a needle ; after five
weeks sections may be prepared, and b}' making sections each
successive week one will be enabled to observe how the micro-
organisms in the course of time penetrate deeper and deeper
into the tissue and gradually bring about its destruction.
I do not look upon discoloration as an essential phenomenon
of decay, and do not therefore trouble myself about the color of
the dentine in artificial decay. It has appeared to me that where
nitrogenous substances were present the discoloration appeared
sooner than if only carbohydrates were used. A decak-ified
tooth placed in a mixture of saliva and meat will become discol-
ored in a few days or weeks.

CARIES OF ANIMAL TEETH.
It is commonly believed that dental caries either does not
occur at all in animals, or at best so seldom that the few cases
which may have been observed are to be regarded as striking
exceptions. And indeed we must grant that the teeth of animals,
compared to those of modern civilized races, are relatively seldom
attacked by caries. But if we compare the teeth of certain
kinds of animals Avith those of uncivilized Imnian races which