is a tendency through the force of habit and association for the same
movements to be performed, though they may not then be of the least
use. Some actions ordinarily associated through habit with certain
states of the mind may be partially repressed through the will, and in
such cases the muscles which are least under the separate control of
the will are the most liable still to act, causing movements which we
recognize as expressive. In certain other cases the checking of one
habitual movement requires other slight movements; and these are
likewise expressive.
II. _The principle of Antithesis_. Certain states of the mind lead to
certain habitual actions, which are of service, as under our first
principle. Now when a directly opposite state of mind is induced, there
is a strong and involuntary tendency to the performance of movements of
a directly opposite nature, though these are of no use; and such
movements are in some cases highly expressive.
III. _The principle of actions due to the constitution of the Nervous
System, independently from the first of the Will, and independently to
a certain extent of Habit_. When the sensorium is strongly excited,
nerve-force is generated in excess, and is transmitted in certain
definite directions, depending on the connection of the nerve-cells,
and partly on habit: or the supply of nerve-force may, as it appears,
be interrupted. Effects are thus produced which we recognize as
expressive. This third principle may, for the sake of brevity, be
called that of the direct action of the nervous system.
With respect to our _first Principle_, it is notorious how powerful is
the force of habit. The most complex and difficult movements can in
time be performed without the least effort or consciousness. It is not
positively known how it comes that habit is so efficient in
facilitating complex movements; but physiologists admit[102] that the
conducting power of the nervous fibres increases with the frequency of
their excitement. This applies to the nerves of motion and sensation,
as well as to those connected with the act of thinking. That some
physical change is produced in the nerve-cells or nerves which are
habitually used can hardly be doubted, for otherwise it is impossible
to understand how the tendency to certain acquired movements is
inherited. That they are inherited we see with horses in certain
transmitted paces, such as cantering and ambling, which are not natural
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