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226 PATHOLOGY OF THE HABD TISSUES OF THE TEETH.

margin, and decay will cease before penetration of the enamel
has occurred. Such areas will become brown or black with the
coming of immimity to the region, and remain as dark spots in
the enamel. They should not be disturbed. When they begin
late, the free margin of the gum has shortened so that the begin-
ning is very close to the gingival line, which soon becomes
involved if neglected. When they occur late in life — at from
forty to sixty — they most frequently begin just at the gingival
line, and often, where there has been some recession of the gums,
they begin in the cementum. In old age they usually begin in
the cementum. It follows, therefore, that the position of the area
of liability in this class of cavities differs considerably at differ-
ent ages of the patient, being well removed from the gingival line
toward the occlusal in the young, close to the gingival line in the
adult, and includes the gingival line in middle life and old age.
At all ages, however, the area of liability is limited toward the
gingival by the free margin of healthy gum tissue; toward the
occlusal by the limit of habitual uncleanliness of the surface, and
toward the mesial and distal by the margins of the surface, or
the angles of the tooth. In the most highly susceptible persons,
the superficial beginning of decay may pass the angles of the
tooth and join with the area of liability of the proximal surfaces.
This never occurs except. in cases of unusual neglect.
The first appearance of decay in the enamel is usually in
the form of a narrow strip of whitening and softening close to,
and following, the curve of the free border of the gum margin,
or just at that point of the surface that is least perfectly cleaned
by the friction of mastication. The extent of the area of begin-
ning decay will correspond with the area of uncleanliness of the
surface. Occasionally this is quite broad occluso-gingivally, but
more generally it is narrow, and not infrequently it is reduced
to a mere line less than a millimeter in breadth. Mesio-distally,
or along the border of the gum, this line varies much in length,
but is rarely less than one-third the breadth of the surface, more
often occupies two-thirds the breadth, and occasionally extends
from the mesial to the distal angle of the tooth. The first pene-
tration of the enamel is usually central to this area, but not infre-
quently extended in a line of minute breaks, which unite, as they
enlarge, forming an elongated cavity. Unless cavities have
gained considerable size before they are seen, there is always a
streak of softened enamel running away from the cavity to the
mesial and to the distal, in which the decay has not yet reached
the dentin.