were at first performed consciously, have become through habit and
association converted into reflex actions, and are now so firmly fixed
and inherited, that they are performed, even when not of the least
use,[114] as often as the same causes arise, which originally excited
them in us through the volition. In such cases the sensory nerve-cells
excite the motor cells, without first communicating with those cells on
which our consciousness and volition depend. It is probable that
sneezing and coughing were originally acquired by the habit of
expelling, as violently as possible, any irritating particle from the
sensitive air-passages. As far as time is concerned, there has been
more than enough for these habits to have become innate or converted
into reflex actions; for they are common to most or all of the higher
quadrupeds, and must therefore have been first acquired at a very
remote period. Why the act of clearing the throat is not a reflex
action, and has to be learnt by our children, I cannot pretend to say;
but we can see why blowing the nose on a handkerchief has to be learnt.
It is scarcely credible that the movements of a headless frog, when it
wipes off a drop of acid or other object from its thigh, and which
movements are so well coordinated for a special purpose, were not at
first performed voluntarily, being afterwards rendered easy through
long-continued habit so as at last to be performed unconsciously, or
independently of the cerebral hemispheres.
So again it appears probable that starting was originally acquired by
the habit of jumping away as quickly as possible from danger, whenever
any of our senses gave us warning. Starting, as we have seen, is
accompanied by the blinking of the eyelids so as to protect the eyes,
the most tender and sensitive organs of the body; and it is, I believe,
always accompanied by a sudden and forcible inspiration, which is the
natural preparation for any violent effort. But when a man or horse
starts, his heart beats wildly against his ribs, and here it may be
truly said we have an organ which has never been under the control of
the will, partaking in the general reflex movements of the body. To
this point, however, I shall return in a future chapter.
The contraction of the iris, when the retina is stimulated by a bright
light, is another instance of a movement, which it appears cannot
possibly have been at first voluntarily performed and then fixed by
habit; for the iris is not known to be under the conscious control of
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