A bird flaps its wings after its head is cut
off, and yet the seat of consciousness and will is certainly in the
brain. A patient with a "broken back," and paralyzed in his legs,
will draw up his feet if they are tickled, although he is entirely
unable to move them by any effort of his will and has no
consciousness of the irritation.
The physiological action is in this case clear. The vibration of the
nerve caused by the tickling travels from the foot to the
appropriate centre in the spinal marrow, and here gives rise to, or
is switched off as, a motor impulse travelling back to the muscles
of the leg, causing them to contract. In the injured patient the
nervous impulse cannot reach the brain, the seat of consciousness,
and hence this is not awakened. Normally consciousness does result
in a majority of such cases, but only after the beginning or
completion of the appropriate action. Yet the movements of our
internal organs, intestine and heart, go on continually, and in
health we remain entirely unconscious of their action.
But reflex actions may be anything but simple. We walk and talk, and
write or play the piano without ever thinking of a single muscle or
organ. Yet we had once to learn with much effort to take each step
or frame each letter. Thus actions, originally conscious and
intended, easily become reflex; often repeated the brain leaves
their control to the lower centres.
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