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270 THIRD PERIOD-MODERN TIMES ^

particles which compose these fibers, gives rise to their destruction. The
causes from which these disorders derive may be external or internal.
The external causes are blows, violent efforts made by the teeth; the
improper use of the file, the application of acids or of other substances
that injure the enamel, alteration of the saliva, impressions of heat or
cold, and also certain kinds of nourishment. Blows or violent efforts
may produce caries, according to the writer, by occasioning the effusion
of the liquid contained in the vessels. The author gives analogous
explanations for the other external causes. As to the internal causes,
thev consist, he says, in alteration of the blood and of the humors.
The teeth, says Fauchard, are more subject to caries than all the
rest of the bones in the human body, because, their tissues being denser,
the vessels are on this account closer together and more easily liable to
be obstructed, choked up, and broken. Besides, the position of the teeth
exposes them more than the other bones to the mimediate action of external
causes capable of producing the disorders alluded to; and finally, what
demonstrates the dental caries to be produced, for the most part, by
external causes, is that false teeth, either human or formed from those
of animals, sometimes become carious just in the same way as the natural
ones; which evidently happens by the sole action of external causes.
It is undeniable that the ideas expressed by Fauchard on the patho-
geny of caries, cannot hold good against criticism. Nevertheless, we
owe a great deal to this author for having once for all put an end to the
ridiculous theory of dental worms, and for having tried to find a reason-
able explanation of the manner in which caries is produced.
The teeth, says Fauchard, have not all the same disposition toward this
morbid process; indeed, notable differences are to be observed in this
respect. The molars are, in fact, more apt to become decayed than the
incisors or the canines; and the upper incisors and canines are more
subject to this disease than the inferior ones, because, by reason of their
position, they are more frequently uncovered and more exposed to heat
and cold, whether in eating and drinking or whether in the mere aspira-
tion or expiration of the air. It is to be observed, besides, that when
the eruption of the last molars is considerably delayed they easily decay.
Having very frequently observed the symmetrical decay of correspond-
ing teeth on both sides of the same jaw, Fauchard considers that these
cases are not simply accidental, but rather holds that the fact depends
on a special cause, which, however, is not easy to determine. He offers,
at any rate, a sufficiently good explanation when he says that as certain
morbid causes (bad humors, etc.) must affect both sides of the mouth
identically, it is but natural that the effects of such causes should be

' Pafie 149.