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192 PRKLIMINARY PREPARATION OF CAVITIES, ETC.

reason that the well-known salutary and protective changes of structure
go on coincidently with the slow inroad. The slight irritation of slowly
advancing caries to some extent exerts a stimulating influence toward
inducing tubular deposits. On the other hand, when the carious pro-
cess progresses with ra})idity, re])arativc etforts upon the ]iart of the
pulp are paralyzed, the organic elements of the tissue become denuded
to a greater extent, and therefore sensitivity is increased to a propor-
tionate degree.
As these fibrillar elements are the means of extending the irritation
to the j)ulp of which they are the peripheral prolongations, it is evident
how important a foctor the active advance of caries is, and also to what
extent the rapidity of the process increases the morbid concomitants
of dental caries. It has been pointed out that the area of hypersensi-
tiveness generally pertains to a narrow line at the outer limit of the
dentin, but in rapid caries this line is a broader one.
The anatomical element of the dentin concerned with its sensi-
tivity is contained within the tubuli. AVhile the exact nature of the
matter in these tubules has not yet been certainly determined, it has
been shown to have sufficient consistence to permit of extension, as
in separating sections under the microscope what apjjcar to be fibers
have been seen. Also the same appearance has been presented in fresh
specimens when the pulp has been drawn away from the dentin. It
is not difficult in reviewing these facts in connection with the various
conditions and phases of dentinal sensitivity to conclude that the exalta-
tion is inseparably connected with an irritated state of the tubular con-
tents. The variation in the degree of sensitivity of different teeth of
the same mouth—of those which are side by side and in a similar
degree of progress of carious action ; the profound fact, heretofore stated,
that the dentin at a short distance beneath the decay is much less sen-
sitive ; that in some instances sedatives modify the degree of pain, and
that coagulants produce a marked impression upon the capacity of the
tubular contents to convey sensation, force by inference the conclusion
that in diseased conditions this anatomical element is largely concerned
in conveying impressions to the central organ of the tooth.
It is also undoubted that unusually high sensitivity of dentin is an
inherent constitutional condition with some persons, and that it pertains
to some families apparently as an inheritance, but may be explained in
these instances as the transmission of acute nervous impressionability.
In connection with this subject should be considered the further
observation that the temperature sense of the teeth is various ; that with
some the application of ice makes no impression upon the teeth when
in normal condition, M'hile with others in the same condition the least
cold induces pain. It would further appear that the degree of sensitivity