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CABIES OF ENAMEL. 89

become entirely careless as to artificial cleaning. The study of
this phase of the spreading of caries and its limitations is of the
utmost importance. The student and the practitioner should be
continually studying it in the mouths of his patients, making out
carefully areas of spreading decay and their normal limitations
under the conditions which he finds. This, if carefully carried
out, will serve as a guide of great importance in the extensions
necessary in the preparation of cavities to prevent recurrence
of decay about cavity margins.
In the consideration of the beginning of caries of the enamel
with reference to treatment, it may be stated, as a fundamental
proposition, that the nidus of each beginning of caries will he
found at that particular point on the surface of the tooth
attacked, or that may he attached in the future, that offers the
best position for the lodgment and undisturbed growth of colo-
nies of microorganisms. These colonies and the caries will
spread superficially on the surface to those lines where their
further spread is limited, (1) by the position of normal gum
tissue; (2) by abrasion of mastication by excursions of food
through the embrasures; or (3), by artificial cleaning. When
this nidus is destroyed by the falling away of the enamel rods
because of caries of the dentin, before such spreading has
reached its limit, as often happens, the conditions are usually
so changed that the growth of colonies on the surface of the
enamel is prevented. This at once limits further superficial
spreading buccally and lingually on the lines thus far illustrated.
Further superficial spreading gingivally, may or may not occur,
as will be illustrated later. The lateral superficial spreading
seems to be stopped by the frequent catching of stringy foods
on the roughened area and the worry of the parts in its removal,
or by the frequent forcing of these lodgments further to the
gingival at each meal time. In many of the cases the loss of the
central and best attachment of the organisms may of itself be
sufficient to produce this effect. However this may be, much
careful observation shows that after the opening of the cavity
this stoppage occurs in many cases. But if a good and perfect
filling is made, restoring the original form and conditions with-
out reference to the possible further spreading of the growth of
colonies of microorganisms, such filling will reestablish the nidus
for growth and this will recur on the filling and extend to the
full limit that it might have done if the original nidus had not
been destroyed, thus reestablishing caries beyond the mar^ns
of the filling. In this way, a well-made filling in the mechanical
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