_expressions_ are just as important as our _impressions_. By
expressions we mean the motor consequences of our thoughts, and in
study they usually take the form of speech and writing of a kind to be
specified later.

The far-reaching effects of motor expressions are too infrequently
emphasized, but psychology forces us to give them prime consideration.
We are first apprised of their importance when we study the nervous
system, and find that every incoming sensory message pushes on and on
until it finds a motor pathway over which it may travel and produce
movement. This is inevitable. The very structure and arrangement of the
neurones is such that we are obliged to make some movement in response
to objects affecting our sense organs. The extent of movement may vary
from the wide-spread tremors that occur when we are frightened by a
thunderstorm to the merest flicker of an eye-lash. But whatever be its
extent, movement invariably occurs when we are stimulated by some
object. This has been demonstrated in startling ways in the
psychological laboratory, where even so simple a thing as a piece of
figured wall-paper has been shown to produce measurable bodily
disturbances. Ordinarily we do not notice these because they are so
slight, sometimes being merely twitches of deep-seated muscles or
slight enlargements or contractions of arteries which are very
responsive to nerve currents. But no matter how large or how small, we
may be sure that movements always occur on the excitation of a sense
organ. This led us to assert in an earlier chapter that the function of
the nervous system is to convert incoming sensory currents into
outgoing motor currents.

So ingrained is this tendency toward movement that we do not need even
a sensory cue to start it off; an idea will do as well. In other words,