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6 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
tion of hair is imperfectly formed, and when in the process of
growth this section arrives at the surface of the skin and is sub-
jected to bending, it breaks and the hair suddenly falls away
These are common phenomena following severe cases of typhoid
fever. The hair follicles are not injured and the hair is replaced
by the regular process of growth.
In the continuously growing teeth of the rodents such an
injury would be finally removed and remedied in the same way
as the grooving seen upon the finger nails, but this can not occur
in the human teeth.
The deformity, though much varied in different cases, is,
when closely analyzed, always similar in character. It always
consists in a failure of the formation or an imperfect formation
of some specific portion of the tooth and of several teeth together.
The portion of the several teeth affected is always that portion
of each that was in process of formation or growth at the same
period in the person's existence. To understand this well one
should study closely the calcification of the crowns of the teeth
and the contemporaneous lines of calcification of the different
teeth. This will be more fully explained in considering the histo-
logical changes occurring in atrophy. In the incisors the de-
formity is oftenest seen in the form of a groove, smooth or pitted,
running across the labial surface from mesial to distal, and close
inspection will generally show that it encircles the tooth com-
pletely, though it is most prominent upon the labial surfaces
where the enamel is thickest. It is seen more often on the incisal
half of the length of the crown. It may be near the cutting edge
of the tooth or anywhere from that point toward the gingival
line. It is also found occasionally in the roots of extracted teeth.
There may be a single groove or pitted line, or there may be two
or even three or more of these. The teeth affected are the inci-
sors, cuspids and first molars of the permanent set, and very
rarely the first bicuspids. If it is very close to the cutting edge
on the central incisors it may not appear on the laterals, but the
occlusal surfaces of the first molars will be atrophied. This is
because these parts of these teeth are in process of formation
at the same time. If the groove is a little further removed from
the incisal edge of the centrals, the lateral incisors will also be
similarly affected. If it is a little higher still, the four incisors,
upper and lower, the cuspids and the first molars will be affected,
but the bicuspids will be free from injury. It is exceedingly rare
that the bicuspids or the second or third molars are affected by
atrophy, for the reason that the enamel and dentin of these teeth
generally have not begun to form until after the age at which
these effects are most liable to occur. But few cases occur in