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[2] "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 to them,--
in the pointing of young pointers and the setting of young setters--