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50 MACROSCOPIC ANATOMY OF THE HUMAN TEETH.

The transverse diameter of the crowns of the upper molars is tlie
long;est.
The LOWER MOLARS ((/) are similar to the ))ermaiient molars in pat-
tern, but are more irregular as to the contour of the morsal surface (/).
The tubercles may be higher than in the upper molars, and the tri-
angular ridges more marked. The (central fossa may be large and wide,
or divided by the triangular ridges. The second molar is five-lobed,
unlike the second permanent molar, which has but four cusps. The
morsal foce is d(H!idedly trapezoidal in outline, the mesio-distal diameter
being greater than the transverse.
The roots of the deciduous molars are similar to those of th(> other
molars, except that they are very divergent to accommodate the crown
of the advancing bicuspids. They are thin and long, and difficult to
enter and fill. The pulp chamber is large and open in the crown ; as
a consequence of this caries soon reaches the pulp. Treatment and
filling of the canals is difficult and uncertain.

The Variations of Tooth Forms.
19. The teeth may vary quite extensively from the typal forms which
have been described, and these variations may be due to a number of
causes. Through all degrees of variation, however, the type is still pre-
served, unless the tooth form is quite destroyed by pathological causes.
The general causes of variation may be enumerated as follows :
(1) Incompleteness of development.
(2) Reversion to primitive types.
(3) Temperamental impress.
(4) Pathological lesions.
(1) Under incompleteness of development may be grouped all
those varieties of stunted growth which are the eifect of disuse and
the consequent effi^rt of Nature to reduce and suppress the teeth as
useless parts. The third molar teeth suffer most from these suppressive
attempts of Nature in the effort toward economy of growth ; next to
these teeth, the upper lateral incisors are most frequently affected by
reduction of size, stunted growth and sup})ression. Other teeth are
rarely affected, or but very slightly, by this influence, except in rare
cases.
(2) Under the second head, reversion to primitive types, we have a
variety of interesting phenomena in the form of parts of the human
teeth which seem to be a zoological legacy. These consist of conspic-
uous features which reappear and seem to recall forms of the teeth
observed in some of the lower animal orders, especially the quadrumana
and insectivora.
Among these features may be mentioned the curved upper central