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CAKIES AS A WHOLE. ITS CLINICAL EEATITBES. 95

avoiding cutting through its recessional line. This recessional
line is usually nearly a direct line from the point of the cusp
to the present location of the point of the horn of the pulp. Where
the point of the horn may be located along that line in any indi-
vidual case can not be told in advance, but that it will not be
much out of that line is a certainty. When it is possible, cutting
that line should be avoided.
In this group of pictures, and in the next following, the
clouded areas stretching away from the areas of decay toward
the pulp are particularly prominent. The best examples are in
Figures 110, 113 and 117. It is an almost constant appearance
in some form, even in the freshest decays, after the acid has
begun to dissolve the calcium salts of the dentin, but it never
appears before the enamel has been penetrated. It was first
described by John Tomes and by him was called the hyaline area
in advance of actual caries. It finally became known as the
hyaline area of Tomes. At first Mr. Tomes supposed this was
caused by an increased deposit of calcium salts, filling the den-
tinal tubules. While the chemico-vital theory of caries was held,
this was looked upon as an effort to bar the further progress
of caries by building against it. With further studies, this expla-
nation became untenable. It now seems more probable that it
is caused by the death of many of the dentinal fibrils. The
appearance differs much in different cases. It may be either
a cloud fringed with white, or a white area fringed with cloud,
an irregular distribution of these, or it may be wholly of the
one or the other. It appears to the best advantage in photo-
graphs of split teeth by reflected light, and its prominence in the
photograph depends much upon the condition of the dentin as to
translucence at the moment of photographing. A freshly cut
tooth is apt to show it best, and, if it is photographed at once
on being removed from water, the surface being simply wiped
dry, the appearance will be much stronger than it will be if
the tooth has been allowed to dry. Therefore, it has happened
occasionally in the pictures presented that the photograph of
one of the halves of a tooth has shown this strongly, while the
other half of the same tooth has scarcely shown it at all. How
much actual injury to the tooth is done by that influence which
causes this hyaline zone can not well be told. That there is a
distinct injury is certainly true. It seems now that the most
rational explanation is that in this zone many of the dentinal
fibrils are dead from the irritation caused by the progress of
caries. If this is true, the extension of this to the pulp may be