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NORMAL CONTOUR IN RELATION TO CARIES. 263

dependence and relation to one another, and the part played by the fluids
of the month in the causation of caries under both original and changed
conditions, had not previously been carefully inquired into. Under the old
belief that contact caused decay it was thought that decay upon approxi-
mal surfaces always began at the point of contact and that this was due
to the fermentative changes occurring in food debris retained at this
point. Investigation proved, however, that the points of contact be-
tween teeth were not only free from decay, but more or less polished
from slight motion of the teeth in their sockets, and that approximal
decay always began just above the contact point, that is, slightly nearer
the gum ; also that it could occur nearly as readily without the presence
of food as with it.
It was further noted that the normal contact of teeth on their
approximal surfaces, which was formerly supposed to be essential only
for mutual support, was equally necessary for the protection of the
tender gum tissue from injurious pressure of food in mastication.
Finally it was observed that those portions of the crown of a tooth
that were beneath the gum margin or those above it that were constantly
covered by saliva (as on the approximal surfaces near the gum) were
always free from the beginnings of decay, and that the approximal and
buccal or lingual surfaces, when faultless in structure, were first attacked
by caries on a line corresponding with the point to which the fluids of
the mouth usually rose. An explanation of this peculiarity was soon
found in the fact that the saliva is usually alkaline and consequently
protective of the parts covered by it, but at its surface, in a state of
rest (as in sleep), this condition of alkalinity is changed to one of
acidity—the calcium salts are dissolved and decay is begun.
As a result of the foregoing observations and investigations it
became apparent to the mass of conscientious workers in the field of
operative dentistry : 1st. That the natural form or outline of each tooth
was the best for its particular function, and that to materially alter it was
to lessen its usefulness and hasten its loss. 2d. That contact of ad-
joining teeth was essential both to the comfort of the individual and
the durability of the organs, 3d. That inasmuch as the teeth originally
decay in spite of their natural form and contact, some plan would have
to be devised by which, in their repair after decay, liability to a recur-
rence of caries would be greatly lessened if not entirely prevented.
To fulfill these requirements there was but one course left to pursue,
namely, to fill approximal cavities in such a way as to restore the
original contour of the surface, and, in all cases where the extent of
decay was sufficient to warrant it, to extend the cavities so far over nj)on
the buccal and liny-ual surfiices as to briup; the enamel mary;ins within
the range of protective influences.