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DISTAL OCCLUSION. 685
teeth of the opposite jaw. The buccal cusps of the lower teeth close
between the buccal and lingual of the upper, and the lingual cusps of
the upper close between the lingual and buccal cusps of the lower.
As the lower jaw moves laterally during mastication the cusps of the
bicuspids and molars grind upon each other, while the six anterior teeth,
overlapping but not touching, pass bv each other and escape wear. In
order to touch the cutting edges of the upper and lower incisors upon
each other the lower jaw is protruded, and at such a time the masticat-
ing teeth do not occlude.
Mal-occlusion may be of many kinds and degrees, to which no
general description can be given. The eruption of a single tooth
labially or lingually or turned on its axis may disarrange the occlusion
of all the neighboring teeth or even all the teeth of both arches. Like-
wise the loss of a single permanent tooth or its failure to erupt may
have a more disastrous etfect.
There are many cases, however, in which both arches are normal,
yet their relation to each other is such that it results in mal-occlusion.
Tlie nature of such mal-occlusion of the arches as a whole mav be
indicated by the position of the second lower bicuspid, which is the

Fig. 601.

Distal occlusion of the jaws.
key to the occlusion ; according to its position, the occlusion of the
arches may be described as normal, distal, mesial, lingual, or buccal.
If the key tooth, the second lower bicuspid, closes between the
upper bicuspids, with its buccal cusp between their buccal and lingual
cusps, the occlusion, both of the kev tooth and all the others, is normal
(Fig. 600).
Distal Occlusion.—If the key tooth closes posteriorly or back of
the normal position to any degree, even to the width of a bicuspid or
more, the occlusion is distal, as shown in Figs. 601 and 603. This
mal-occlusion may be on one or both sides of the mouth. It may be
due to lack of development of the lower maxilla, or the fault may be
in the temporo-maxillary articulation.