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THE OCCLUSION OF THE TEETH. 19

Second : The rounded square (Fig. 2, h). Tliis is the medium
arch and is the form usually met with in ordinary, well-developed ro-
bust Americans. The canines seem to be only so prominent as to give
character to the arch without a resemblance to the arches of the lower
animals. The incisors are vertical and the line curves slightly from
one canine to the other. The bicus})id-and-molar line curves slightly
outward from the canine and converges at the rear.
Third : The ROUNDED ARCH (Fig. 2, c). This is the circular or
** horse-shoe " arch. It is nearly semicircular, the ends curving in-
ward at the rear, the outlines of the arch tracing a decided horse-shoe
shape. The canines are reduced to the level of the arch, so that there
is no prominence of these teeth. The bicuspids and molars follow the
line of the curve. This arch is quite characteristic in some races, as
the brachycephalic South Germans.
Fourth : The rounded V (Fig. 2, d). In this form the round arch is
constricted in front or narrowed so that the incisors mark a small curve
whose apex is the centre. It is the arch of beauty and is that most
admired in women of the Latin races.
These are but the basal forms of the dental arch. Ordinarily, mod-
ifications of these types occur in all degrees ; it is the variations, the
composites, which are most met with.
5. The Occkision of the Teeth.—The upper teeth describe the seg-
ment of a circle larger than that of the lower teeth ; so that the edges
of the anterior teeth above close over those below, and the buccal cusps
of the grinding teeth above close outside of the buccal cusps of the
lower teeth (Fig. 3). By this arrangement the buccal cusps of the
lower grinders are received into the de-
Fig. 3.
pressions or sulci between the buccal and
Incisors. Bicuspids. Molar.'
lingual rows of the cusps and tubercles
of the superior molars and bicuspids, and
the lingual cusps of the upper grinders
are received into the sulci of the lower
grinders. By this arrangement the whole
of the morsal surfaces of these teeth are
brought into contact in the several move-
ments of mastication, thereby rendering
the performance of this function more
effective.
Then, again, the upper incisors usually The relative position of tlie upper
and lower teeth in occlusion.
close over the lower for one-third of their
length. This allows of the shearing action by which the incisive func-
tion is performed as the edges of these teeth are drawn past each other.
The line of the horizon of occlusion (Fig. 4, ^1-^) presents a decided