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THE HISTOLOGICAL STRUOTUKE OP THE TEETH. 101

appears as a fine powder. This whitened appearance may be
due to caries of the enamel. In this case, all of the whitened part
must be cut away. If the enamel is sound, by continuing the
shaving motion, it will Ijecome clear and have a firm, vitreous
appearance. This condition of the enamel wall should always
be obtained in finishing the preparation of the cavity.
Rarely there are white spots in the enamel because of imper-
fect development (See Figures 46, 47, Volume I) that may be
included in the area of the cavity or appear on the line upon
which the enamel margin should be found. In the latter case it
will become a question whether to cut them out or leave them.
When these are covered in with a good glazed surface, it is per-
fectly safe to leave them, provided a good filling can be made
without breaking the margin. Generally this may be done;
though in some cases they should be cut away. These white
spots must be distinguished sharply from caries of enamel which
also may show as white spots. Both cut more easily than normal
enamel. Generally the white spots due to faulty development
have a glazed surface; carious spots do not. Often caries of
the enamel occurs as a considerable area of whitening on several
teeth, particularly on buccal surfaces in which the enamel rods
have been separated to a considerable depth, as shown in Figure
110. This is often spoken of as chall^ enamel. The condition
has been produced by caries.
In the study of the enamel in sections, it has been seen that
the lines of the grooves are weak lines on account of the imper-
fect fusion of the enamel plates in coming together during devel-
opment. This is true even in those grooves that are most
perfectly closed. But in very many cases they are imperfectly
closed, so that along these lines the enamel has no strength.
Also when cutting near to a groove, the enamel rods incline
toward the groove, increasing the danger of leaving short ends
of rods at the cavo-surface angle.
Rule : When in the preparation of a cavity, the line of the
cavity margin must approach near a groove, cut past the groove.
This rule should be regarded as applying in all positions
upon the teeth if the cavity margin is parallel, or very nearly
parallel, with the groove. For instance, in preparing large cavi-
ties in the distal surfaces of upper molars, the lingual portion
of the disto-lingual groove is often approached. In such cases,
the enamel should be cut away, including the groove, to and over
the ridge onto the lingual surface, where its direction is more to
the mesial. Then break away from it rather suddenly, forming