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262 PATHOLOGY OP THE HARD TISSUES OF THE TEETH.

InTERCTJSPIKG OF THE FlEST PERMANENT MOLARS.
At the risk of going somewhat beyond the field intended in this
book, I must call attention to what I will designate as the proper
intercusping of the first permanent molars when they first come
into occlusion. The teeth are formed so that the teeth of the lower
jaw will intercusp with the teeth of the upper jaw in a certain
way, which is very clearly shown in the photograph, Figure 101,
of this volume. Also the forms of the teeth are such that if they
miss the exact position in which they should occlude a little way
only, as they come into occlusion, the tendency is to slide on the
slopes of the cusps in such a way as to bring them to the exact
relative positions designed— a most beautiful provision of
nature for correcting slight deviations that should be closely
studied until its meaning is clearly understood. When, however,
the teeth are so much out of normal relation at the time that
the teeth first make contact in coming into occlusion that the
points of the cusps overstep each other ever so little, the wrong
slopes become the moving force directing them into an increased
abnormality of position. That is to say, if the mesio-buccal cusp
of the upper first molar should strike the point of the mesio-
buccal cusp of the lower first molar in such a way that its distal
slope instead of its mesial slope slides on that cusp, the upper
molar will be pushed to the mesial instead of the distal and the
abnormality will be increased instead of being corrected. In
that case, the upper first molar will be one full cusp width too
far forward in its relation to the lower first molar. This error
will then be forced upon the other teeth, crowding the front
part of the upper arch so that there will be either protrusion
of the upper front teeth or irregularity among the bicuspids
or cuspids as a result. On the other hand, the case may be
reversed. In the relation of these teeth to each other as they
approach in coming into occlusion, the lower first molar may
be quite a little forward of its normal relation to the upper,
but so long as the point of the mesial cusp of the upper strikes
ever so little to the mesial of the point of the disto-buccal cusp
of the lower first molar, the sliding on the slopes of the cusps
will correct the malposition. If, however, the position of the
point of the mesio-buccal cusp of the upper first molar should
be ever so little to the distal of the point of the disto-buccal
cusp of the lower first molar, the sliding will increase the mal-
position instead of correcting it. The result will be an irregu-
larity or a protrusion of the lower teeth. In this the buccal