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8 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.

lobe lias failed of formation, resulting in this peculiar scar, but
more recent investigation seems to show that the whole incisal
edge has failed in most of these cases, and that the angles of the
tooth have been drawn together over the injury, giving the out-
line of the tooth this rounded appearance. Certainly many of
these teeth are much shorter than normal. Generally an ugly
deformity of the occlusal surfaces of the first molars accompanies
this type. In the molars little spiculse of cusps are likely to be
sticking up much too close together, while the rest of the occlusal
surfaces are much too small, crumpled together and sunken into
the crown, which, other than this, will be of full size and form.
These teeth decay quickly in case there is a tendency to caries in
the individual.
Of the incisors shown in this illustration, only the centrals
are affected. The calcification of the cutting edges of these is
occasionally just begun at birth, and if not begim then, is usually
begun within one year. The injury, therefore, occurs soon after
the birth of the child from some cause which interferes with
nutrition. A very curious fact in pathology is rendered promi-
nent in this form of atrophy. It is this : when the nutrition of
any single part of the enamel organ is so impaired that its func-
tion is stopped or very seriously disturbed, that particular part
does not recover, and no additional enamel is formed by that
part. This will appear more prominently in the histological
specimens. It is for this reason that these teeth have the peculiar
rounded appearance of the cutting edge. The enamel organ
of that part is arrested in its work at the very beginning of the
(ialcification. and therefore the immediate incisal edge fails
entirely. The rest of the organ goes on with its work after the
recovery and the tooth is drawn in over the scar. In the incisors
this form of atrophy is apt to be attacked by decay in this incisal
pit very soon after the teeth have taken their places in the arch.
They should be filled at once if decay is discovered. This par-
ticular form of atrophy is seen less frequently than others.
The occlusal surfaces of the first molars are occasionally
badly atrophied when the incisors have escaped. Usually these
have just begun their calcification at birth, and occasionally the
calcification of the central incisors does not begin for one year
after birth. In such cases a severe illness may injure the molars
and not injure the incisors.
Much the more common forms are those illustrated in Fig-
ures 2 and 3, the ilhistrations showing rather bad cases. In
Figure 2 the deformity is confined to the cutting edges, appar-