Page 26 - My FlipBook

24 MACROSCOPIC ANATOMY OF THE IIUMAy TEETH.

mesial and distal inai;<>'iMs of (his surface. The basal ridii'c {f>) is a
strong- elevation continuous with the marginal ridges at the hase of
the crown. It is sometimes developed into a raised cusp, the ridge
at the base of which forms a cingulum. A ridge or lobe (r) extends
from the basal ridge to the centre of the edge, uniting with the median
lobe from the labial face to form the median tubercle. A dei)ression
or fossa (cif) is found on each side of the median lobe between it and the
marginal ridges, or, when the lobe is low or entirely absent, these fossre
mav be continuous. A fault or fissure at its junction with the basal
ridae forms the seat of caries in teeth of low structure.
The mesial face (Fig. 10) is a rather long triangle in shape, with a
concaved base at the cervix of the tooth (a),
and a long point toward the edge. It is
nearly straight in a longitudinal direction,
but rounded and convex transversely. It is
longer than the distal face, the edge descend-
ing in that direction. The enamel line dips
downward into this face, and there is a de-
Mesial. Distal. 'J pressiou above it (h) which sometimes extends
The mesial and distal fiices and u])ward ou the root. The point of contact
edge of the upper central in- . • . -i • ^.^ ^.^.•
With the opposing tooth is near the cutting
cigoj..
edge.
The distal face is also triangular in outline (Fig. 10) but it is more
curved in the longitudinal axis, so that this surface is convex in all
directions. It is most curved in the transverse direction. The enamel
dii)s downward into the surflice (d), as in the mesial, but there is not so
much of a depression above this line. The point of contact is one-third
of the distance from the angle (e).
The edge, or morsal margin, of the crown is formed liy the com-
pression of the top of the truncated primitive cone. It is quite wide
and square except at the distal corner, which is rounded. The angle
w^itli the mesial face is acute (Fig. 10, /). When the tooth is first
erupted, the edge has three ]:)rominent tubercles (g), which correspond
to the ridges on the labial and lingual faces. These are soon worn off
with use, so that the edge usually looks straight. The pitch of the
edo-e is toward the median line.
The neck of the central incisor is a rounded pear-shape in outline,
the labial half being wider (Fig. 11, a) than the lingual. There is not
much constriction of the tooth at the neck. The enamel edge curves
upward on the root on the labial and lingual sides, and dijis down-
ward on the mesial and distal flices. It terminates abruptly on all
sides, especially on the lingual, where a considerable ridge is some-
times raised (Fig. 10, c).