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40 MACROSCOPIC ANATOMY OF THE HUMAN TEETH.

convex, ncarlv flat at the marginal ridge, and rounded at the cervix,
beino; (l(>j)rcssed toward the lingual root. Sometimes a depression from
the hifui-cation of the mesio-buceal and lingual roots extends part way
up on the face {a).
The (Udal face is similar to the mesial except that it dips more
toward the cervix, and is, perhaps, more rounded toward the lingual
root.
The morsal surface (Fig. 22) is the most important part of this tooth,
and shows features that make it interesting and unique. The abrupt

Fig. 2-2.

d h j c
The morsal surface of the upper first mular.

change from the bicuspid form is notable, for there are presented four
cusps, a doubling of the number ; the outline of this face presents a
square form with tubercles at each corner, the mesio-buccal («), the
disto-buccal (6), the mesio-lingual (e), and the disto-lingual (d) ; the lat-
ter is erratic and may be either pronounced or quite reduced in size.
There are four marginal ridges—the mesial (e), buccal (/), distal (g)
lingual (h), the oblique (t), and the four triangular ridges (_/). The oblique
ridire connects the mesio-ling-ual with the disto-buccal tubercle and is
really the remnant of the marginal ridge of the tricuspid molar ; the
fourth cusp, the disto-lingual, being raised up on the disto-lingual side.
The four triangular ridges descend from the four tubercles toward the
centre of the tooth, the oblique ridge being formed by the fusion of the
triangular ridges of the mesio-lingual and disto-buccal cusps.
There are two fossae (A-), one mesial and the other distal to the oblique
ridge. Sometimes the latter is cut by a groove or sulcus (/) which
extends from the mesial to the distal fossa. Sometimes by the reduction
of the disto-lingual lobe and cusp, the mesial fossa is extended and
becomes central to the crown. A groove extends from the mesial fossa
over the buccal marginal ridge (hi) quite on to the buccal face, dividing
the mesial from the distal buccal lobes. A groove also extends over
the lingual marginal ridge (71) down upon the lingual face, dividing the
lingual lobes. When this groove becomes a fissure, caries ensues and
the disto-lingual cingule readily breaks away, this cingule being a weak
feature in the mechanical design of this tooth ; cutting the distal mar-
ginal ridge also weakens this cusp. The triangular grooves branch from