The distribution of these stresses is further complicated and modified
by such factors as:

1. The completeness or incompleteness of the occiput and the
location and extent of its attachment to the dermal roof.

2. The size and rigidity of the braincase and palate, and
the extent and rigidity of their contact with the skull.

The stresses applied to the cheek fall into two groups. The first
includes all of those stresses that ran through and parallel to the
plane of the cheek initially. The weight of the jaw and snout, the pull
of the axial musculature, and the necessity to provide firm anchorage
for the teeth created stresses that acted in this manner. The second
group comprises those stresses that were applied initially at an
oblique angle to the cheek and not parallel to its plane. Within this
group are the stresses created by the adductors of the jaw, pulling
down and medially from the roof, and sometimes, during contraction,
pushing out against the cheek.

It is reasonable to assume that the vectors of these stresses were
concentrated at the loci of their origin. For example, the effect of
the forces created by the articulation of the jaw upon the skull was
concentrated at the joint between the quadrate, quadratojugal, and
squamosal bones. From this relatively restricted area, the stresses
radiated out over the temporal region. Similarly, the stresses
transmitted by the occiput radiated over the cheek from the points of
articulation of the dermal roof with the occipital plate. In both of
these examples, the vectors paralleled the plane of the cheek bones.
Similar radiation from a restricted area, but of a secondary nature,
resulted from stresses applied obliquely to the plane of the cheek. The
initial stresses caused by the adductors of the jaw resulted from
muscles pulling away from the skull-roof; secondary stresses, created
at the origins of these muscles, radiated out over the cheek, parallel
to its plane.

The result of the summation of all of those vectors was a complex grid
of intersecting lines of force passing in many directions both
parallel to the plane of the cheek and at the perpendicular or at an
angle oblique to the perpendicular to the plane of the cheek.

Complexities are infused into this analysis with the division of
relatively undifferentiated muscles into subordinate groups. The
differentiation of the muscles was related to changing food habits,
increased mobility