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400 DE^'TAL ANATOMY.

The second and third premolars are considerably larger than the first,
and are implanted by two roots, of which the posterior is the larger.
These two teeth resemble one another very closely, the only appreciable
difference being their slight disparity in size. Their crowns, like that of
the first, are of greater longitudinal than transverse extent, and are pro-
duced into a prominent cusp situated a little anterior to the centre. The
posterior ridge is interrupted shortly before it joins the cingulura by a
deep transverse notch which gives rise to a distinct cusp, the jjosferior
baml tubercle, situated over the hinder root. A faint indication of a
second cusp is seen just behind this as an elevation of the cingulum.
The antero-internal tubercle is present, and occupies relatively the same
position as it does in the first premolar. The cingulum is more promi-
nent on the inner than on the outer side of the crown, and Avith the two
ridges encloses a triangular space.
The fourth premolar is by far the largest and strongest tooth of the
premolar series. It is commonly known as the " flesh tooth," or supe-
rior sectorial, for reasons presently to be given. It is implanted by
three roots, two external and one internal. The crown is comjiosed of
tw^o principal lobes supported by the two external roots, and a small
antero-internal one supported by the internal fang The tAvo prin-
cipal lobes have an antero-posterior position, and are separated from
each other by a deep, narroAv fissure. Of these, the anterior is the
larger and higher of the two ; when viewed externally it resembles a
cone with the anterior contour produced. Internally it is flattened
somewhat, so as to correspond with the flattened inner surface of the
posterior lobe. Posteriorly it is produced into a strong blade-like ridge,
which is terminated by the vertical fissure, while its anterior surface is
marked by a moderate vertical ridge. The posterior lobe is essentially
chisel-shaped in form, with the bevelled edge external ; its apex forms
a blade-like crest which extends the entire length of the lobe. The
internal lobe is small, and occupies a position at the antero-internal
angle of the croAvn, being connected w-ith a faintly-marked cingulum
which surrounds the base of the crown. When we attempt to homol-
ogize the component lobes of this tooth with those of the premolars' in
advance of it, it is not difficult to see that the anterior lobe is the prin-
cipal cone, that the posterior one is merely an exaggerated posterior
basal tubercle, while the internal lobe is strictly homologous with the
structure which has a similar position in the others. The three anterior
premolars are not in as close contact as the teeth in the back part
of the jaw, but are separated from each other by slight intervals, which
are most conspicuous between the first and second.
The premolars of the lower jaM" are similar in form to those of the
upper, with the important exception of the fourth or last, wherein
there is to be found a wide difference both in size and structure.
The first of the lower series is smaller than the corresponding tooth
above, and has a simpler, more conical crown. It is separated by a
considerable diastema from the canine in front of it, but is almost in
contact with the second behind. The second and third resemble those
"vvhich are in a like position in the upper jaw, while the fourth is also
similar to the corresponding tooth above, with the exception of a