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the face; then the slightest smile deforms the countenance, by the
unequal action of the muscles; on that side where the nerve is
entire in the performance of its office, the act of smiling takes
place, while the muscles of the other remain inactive, and are
drawn into distortions.
In the former edition of these essays I had shown the number
and complication of the muscles used as instruments of expression;
but by these discoveries of distinct properties in the nerves, we see
why there is a complication of nervous branches, in proportion not
merely to the number of muscles to be called into action, but to
the variety of uses to which they are put—the various combinations
which they form, in alliance with different organs. It now appears,
that by an apparatus of appropriate nerves, the muscles of the face,
neck, and chest are drawn to co-operate in the act of respiration.
But by these observations it is also proved, that it is through the
nerves of respiration that the muscles become agents of expression ;
for although they can both act and feel after the respiratory nerves
are cut, they no longer express passion, but remain tranquil during
the utmost excess of suffering or of passion in the animal. When,
therefore, we shall have proved that the organs of respiration are
the organs of expression, as well as of speech, the mystery that
hangs over this subject will disappear, and the motions of the
countenance and of the frame of the body will be as intelligible as
the natural expression of the voice.