ard, this
voluntary movement does not have the suddenness and quickness of the
true reflex. For all that, the cerebrum can exert an influence on the
knee jerk. Anxious attention to the knee jerk inhibits it; gritting
the teeth or clenching the fist reinforces it. These are cerebral
influences acting by way of the pyramidal tract upon the spinal center
for the reflex.

Thus the cortex controls the reflexes. Other examples of such control
are seen when you prevent for a time the natural regular winking of
the eyes by voluntarily holding them wide open, or when, carrying a
hot dish which you know you must not drop, you check the flexion
reflex which would naturally pull the hand away from the painful
stimulus. The young child learns to control the reflexes of
evacuation, and gradually comes to have control over the breathing
movements, so as to hold his breath or breathe rapidly or deeply at
will, and to expire vigorously in order to blow out a match.

The coughing, sneezing and swallowing reflexes likewise come under
voluntary control. In all such cases, the motor area facilitates or
inhibits the action of the lower centers.

Super-motor Centers in the Cortex

Another important effect of the motor area upon the lower centers
consists in combining their action so as to produce what we know as
skilled movements. It will be remembered that the lower centers
themselves give cooerdinated movements, such as flexion or extension of
the whole limb; but still higher cooerdinations result from cerebral
control. {56} When the two hands, though executing different
movements, work together to produce a definite result, we have
cooerdination controlled by the cortex. Examples of this are seen in
handling an ax or bat, or in playing the piano or violin. A movement
of a single hand, as in writing or buttoning a coat, may also
represent a higher or cortical cooerdination.

[Illustration: Fig. 15.--(From Starr.) Axons connecting one part of
the cortex with another. The brain is seen from the side, as if in
section. At "A" are shown bundles of comparatively short axons,
connecting near-by portions of the cortex; while "B," "C," and "D"
show bundles of longer axons, connecting distant parts of the cortex
with one another. The "Corpus Callosum" is a great mass of axons
extending across from each cerebral hemisphere to the other, and
enabling both hemispheres to work together. "O. T." and "C. N." are
interior masses of gray matter,