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HYPERSENSITIVE DENTIN. 191

stance changes take place witliin the tubules by which their capacity to
convey sensation is diminished or obliterated as the case may be.
]M\e)i the gum recedes, exposing the cementum, a very high degree of
sensitivity is often excited, which is prone to decline by spontaneous
changes of structure. There is often here the added influence of acid
conditions of the mucous secretions where they flow out upon the teeth
at. this point, and where, too, the parts are not easily cleansed. It is a
notable fact in connection with cervical hypersensitiveness that while it
persists these parts are less liable to decay than when loss of sensitive-
ness here takes place.
The area of hypersensitivity usually is not evenly distributed
throughout the carious cavity, but has its chief seat near the line of
union of the dentin with the enamel, thus bearing out the law that
sensitivity is greatest at the terminal end-organs of the sensory nerves,
with the further qualification that the more minute the fibrillse the
greater may be the acuteness of the sensitivity. This fact is illus-
trated by the example of cavities in the occlusal surfaces of the molars,
whicli manifest pain only at the margins ; is only less evident in the
cavities of approximal surfaces, and is strongly shown in the shallow
buccal and labial cavities, which present their whole surfaces near the
line of juncture of enamel and dentin.
In most cases of caries, the zone of highest sensitivity is iimaediateli/
beneath the soft portion of the decay, and when this layer of dentin is
cut away the pain becomes less, in some instances approaching the nor-
mal. This statement, howev^er, has force only in the milder manifesta-
tions of this condition.
The Effect of Acid Conditions of the Oral Fluids.—In the pre-
vious chapter some allusion was made to the fact that an add state of
the oral fluids is detrimental to the teeth as promoting carious action,
and that alkaline or even neutral states have a retarding influence.
Here it must be considered as an axiom that no cause is so active as a
primary influence in inducing excessive dentinal sensitivity as a con-
stant slightly acid state of these fluids ; and, conversely, that a neutral
or slightly alkaline state is non-irritating. These conditions should be
kept in constant view in dealing with this subject.
The degree of sensitivity of dentin is modified by a variety of
other general conditions. These are—relative density of the structure,
rapidity of the carious action, and the constitutional peculiarities of the
person which are connected most dii-ectly with nervous impressiona-
bility to disturbances of the tissues.
The rate of progress of caries exerts considerable modifying influence
over dentinal sensitivity. AMien caries is of slow progress the amount
of organic tissue exposed to irritation is comparatively small, for the