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THE OCCLUSION OF THE TEETH. 21

arrangement is such tliat the dynamics of mastication is subserved
and the greatest effectiveness secured (Fig. 5). Thus the morsal sur-
face of the upper central incisor is opposed to all of that of the cen-
tral incisor below and to the mesial half of the lateral ; the upper lat-
eral opposes the distal half of the lateral below and the mesial face of
tlie canine ; the upper canine, the distal half of the face of the lower
canine and the mesial half of the first bicuspid; the upper first bicuspid
opposes the distal half of the lower first bicuspid and the mesial half
of the second ; the upper second bicuspid opposes the distal half of
the lower second bicuspid and part of the lower first molar : the. upper
first molar opposes the distal part of the lower first molar and the me-
sial half of the second ; tlie upper second molar opposes the distal half
of the lower second and part of the third ; and the upper third covers
the remainder of the lower third molar.
By this method of apposition the teeth are so arranged that two
teeth receive the impact of half of two of the opposite jaw, thus
distributing the force of occlusion and ensuring the safety and strength
of the teeth. This " break-joint " arrangement permits each tooth to
bear two opposing ones, and also helps to preserve the alignment.

Fig. G.

Incisors. Canines or Premolars or Molars.
cus2)id.i. Bicuspids.
The classes of the teeth, comprising the left half of a full denture.

Then again, if one tooth be lost, the opposing teeth still rest against
two teeth, one at each side of the space. The normal condition of
the articulation is rarely preserved, however, as mutilation usually dis-
turbs it ; the teeth move on account of the force of occlusion, and effec-
tive mastication is more or less destroved.