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 the will in any animal.
In such cases some explanation, quite distinct from habit,
will have to be discovered. The radiation of nerve-force
from strongly-excited nerve-cells to other connected cells,
as in the case of a bright light on the retina causing
a sneeze, may perhaps aid us in understanding how some reflex
actions originated.
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