ion that we see in the scratching
movement of the dog. Such a motor center can be aroused to activity by
a sensory stimulus, and the resulting movement is then called a
reflex.

The lower center can be aroused in quite another way, and that is by
nerve currents coming from the brain, by way of the motor area and the
pyramidal tract. Thus flexion of the leg can occur voluntarily as well
as reflexly. The same {54} muscles, and the same motor neurones, do
the job in either case. In the reflex, the lower center is aroused by
a sensory nerve, and in the voluntary movement by the pyramidal tract.

The story is told of a stranger who was once dangling his legs over
the edge of the station platform at a small backwoods town, when a
native called out to him "Hist!" (hoist), pointing to the ground under
the stranger's feet. He "histed" obediently, which is to say that he
voluntarily threw into play the spinal center for leg flexion; and
then, looking down, saw a rattler coiled just beneath where his feet
had been hanging. Now even if he had spied the rattler first, the
resulting flexion, though impulsive and involuntary, would still have
been aroused by way of the motor area and the pyramidal tract, since
the movement would have been a response to _knowledge_ of what that
object was and signified, and knowledge means action by the cerebral
cortex, which we have seen to affect movement through the medium of
the motor area. But if the snake had made the first move, the same leg
movement on the man's part, made now in response to the painful
sensory stimulus, would have been the flexion reflex.

Facilitation and Inhibition

Not only can the motor area call out essentially the same movements
that are also produced reflexly, but it can prevent or _inhibit_ the
execution of a reflex in spite of the sensory stimulus for the reflex
being present, and it can reinforce or _facilitate_ the action of the
sensory stimulus so as to assist in the production of the reflex. We
see excellent examples of cerebral facilitation and inhibition in the
case of the knee jerk. This sharp forward kick of the foot and lower
leg is aroused by a tap on the tendon running in front {55} of the
knee. Cross the knee to be stimulated over the other leg, and tap the
tendon just below the knee cap, and the knee jerk appears. So purely
reflex is this movement that it cannot be duplicated voluntarily; for,
though the foot can of course be voluntarily kicked forw