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118 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
"self-limiting" is applied to those diseases which run a specific
course and, if the patient has withstood the attack, tend to recov-
ery at about a certain period. In all of these, it is found that
there has been a change in the blood, which renders the person
immune, and this is known as the development of an antitoxin,
which remains more or less permanent in the blood. In some of
the diseases it seems to become a permanent fixture, as in small-
pox. In others, it disappears after a time more or less com-
pletely, and the person again becomes susceptible. In this way,
children may have scarlet fever and some other of the chil-
dren's diseases more than once before they become permanently
immune. In other cases, susceptibility to disease seems to go
and come. At one time a person exposed to a contagious disease
apparently under all conditions that would favor its develop-
ment, goes free. The same person at other times, with probably
otber conditions of the body juices and cells, will take the disease
readily under otherwise similar conditions. Most of the microbic
diseases have some one or more of the peculiarities that have
been mentioned, but there are some that are not in any wise self-
limited. Tuberculosis is one of these. Although purely a
microbic disease, there seems to be no self-limiting effect.'
Something similar to the changes that happen in suscepti-
bility and immunity to other diseases must occur in caries of
the teeth, for to-day it is only upon this ground that we can
explain the conditions of susceptibility and immunity that are
so prominently before us. Formerly, the susceptibility or
immunity of the teeth to decay was differently explained. It
was supposed that hardness or softness of the teeth, the amount
of calcium salts they contained, was in a large degree the con-
trolling factor, and under that supposition, when the teeth of
children were seen to be decaying badly, the interpretation was
that the teeth were soft or poorly calcified. If the child grew
up without decay of the teeth, the interpretation was that the
teeth were hard and firm and for that reason did not decay. It
was on this ground that the effort was made to explain the
variations of susceptibility and immunity of which we have
spoken.
Physical Characters of the Teeth.
The idea that some teeth are hard and some teeth are soft,
grew up in the minds of the dental profession and of the laity
many years ago, and this was generally regarded as a fixed fact.