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38 MACROSCOPTC ANATOMY OF TIIF. HTMAX TKKTII.

\uvr muscles, fur the ])urpose of cnisliiiiu' and masticating; I'ood. Thoy
arc im})ortant factors in alimentation and contribute to the function of
digestion by preparing ibod for the stomach. Their loss imjiairs this
function seriously and leads to derangement of the stomach by over-tax-
ing it with imperfectly masticated food-substances.
15. The Upper Molar.—The typical U])})er molar is formed by the
fusion of three cones, as is plainly observed in the three roots and the
three tubercles (Fig. 21, A). The tricuspid molar, therefore, is a jirimi-
tive form, and is rarely seen in man, the normal form being quadritu-
berculate. The fourth additional cusp, the disto-lingual {b), is merely
a supplemental cusp added to the crown. An upper molar is, there-
fore, composed of three tubercles, and a cingule which has not yet
developed a root to support it. The tritubereulate molar is the primitive
form of this tooth, the quadrituberculate appearing later, and is found in
onlv a iew liviup; forms, as some of the lemurs and the insectivorous
and carnivorous mammalia. In man there is sometimes a reversion of
the upper molar to the tritubereulate form, which is a marketl degeneracy
in the form of this tooth. In an analysis of this tooth, therefore, the
mesio-buccal tubercle (c) is the canine cusp ; the mesio-lingual, the bicus-
pid cusp [d) ; the disto-buccal, the molar cusp {e), and the disto-lingual
is but a supplemental cusp,—it is not a true cusp, as it has no root to
support it.
The architecture of the upper molar presents some interesting features.
We observe that the crown is in a general way a geometrical form, a
wdien perfect and symmetrical. It is suggestive of symmetry,
cube (/),
but when taken with the root form is not quite perfect, for it is sup-
ported on three roots instead of four to correspond with the four tuber-
cles at the four corners. So it lacks the " harmony of adequate sup-
port," which is a cardinal principle in architecture. But the crown
separately is a symmetrical form, a cube, although the angles are rounded
off and the corners and ])oints are toned down and not acute. We no-
tice that there are four strong columns, one at each of the four corners
They are connected on the four sides by the marginal ridges acting
(fj).
as strong connecting arches (h). These arches are related to the col-
umns of the crown, and both are impressively proportioned. The cusps
may be likened to the capitals of the columns, and the descending mar-
ginal and triangular ridges to the cornice, gathered together to form the
capitals. The triangular ridges may be considered girders (i), bind-
ing the four together and also bracing the square obliquely. Or, the
four triangular ridges running to the centre may be regarded as half-
arches or buttresses, supporting the roof vault,—the grinding face.
Other elements could be marked out in an architectural study of the
crown of this tooth, showing its beautiful design and symmetry.