Page 29 - My FlipBook

THE INCISORS. 27

The lateral incisor is very irregular as to form, presenting various
degrees of deformity or abnormality, and may sometimes be reduced to
a mere peg. It is also erratic as to eruption, being sometimes sup-
pressed, not appearing for several generations of a family. It follows
the third molar in the frequency of its irregularities both as to form
and frequency of non-eruption.
The third incisor of the primitive typal mammal sometimes reap-
pears in man, and is kno^vn as a supernumerary. It rarely assumes the
proper incisor form and position in the arch, but usually erupts within
the arch and is a mere pointed-peg-shaped tootli.
9. The Lower Incisors.—These are most conveniently described as
a group, as they are very similar in form, having but slight variations
between the central and lateral incisors to be noted.
They are located in the anterior portion of the lower jaw, upon each
side of the median line, opposite the incisors above. Their function is the
same as that of the upper incisors, the cutting of food, which they per-
form by opposing the upper. The lower central opposes only the cen-
tral above ; the lateral, both the upper central and lateral incisors.
The lower central incisor is the smallest tooth in the dental series.
It is of spade-like form (Fig. 14), the crown being a double wedge
shape («, b). The first wedge {a) is observed on viewing the crown
from the front, the widest portion being
at the morsal edge and the point at the
cervix. The second wedge is observed
from the side {h), the widest part being
at the neck and the point at the morsal
edge of the crown. The edge is thin,
but the labio-lingual diameter increases
rapidly to the cervix, which is the
The lower incisor.
widest pnrt. The crown is widest
mesio-distally at the edge, but diminishes to the neck, which is scarcely
more than half the width of the edge. The tooth cone is therefore
compressed in one direction at the edge, and in another at the cervix.
The mechanical elements are the same as those of the upper central, but
with tlie parts less strongly marked.
The labial face is a long wedge shape {a), the widest part at the
edge and narrowing to the cervix. It is usually straight, or nearly
so, longitudinally, and straight across the edge, but round and con-
vex at the neck and the cervical half. Sometimes vertical ridges are
found on these teeth wlien they are first erupted, but these soon
wear oif.
The UmjuaJ face is depressed and concave from edge to cervix (c),
but less so fnmi side to side. The maroinal ridges are often well