sword, we teach many of our muscles to act together or in succession by
repeated voluntary efforts; which by habit become formed into tribes or
trains of association, and serve all our purposes with great facility, and
in some instances acquire an indissoluble union. These motions are
gradually formed into a habit of acting together by a multitude of
repetitions, whilst they are yet separately causable by the will, as is
evident from the long time that is taken up by children in learning to walk
and to speak; and is experienced by every one, when he first attempts to
skate upon the ice or to swim: these we shall term voluntary associations.

2. All these muscular movements, when they are thus associated into tribes
or trains, become afterwards not only obedient to volition, but to the
sensations and irritations; and the same movement composes a part of many
different tribes or trains of motion. Thus a single muscle, when it acts in
consort with its neighbours on one side, assists to move the limb in one
direction; and in another, when, it acts with those in its neighbourhood on
the other side; and in other directions, when it acts separately or jointly
with those that lie immediately under or above it; and all these with equal
facility after their associations have been well established.

The facility, with which each muscle changes from one associated tribe to
another, and that either backwards or forwards, is well observable in the
muscles of the arm in moving the windlass of an air-pump; and the slowness
of those muscular movements, that have not been associated by habit, may be
experienced by any one, who shall attempt to saw the air quick
perpendicularly with one hand, and horizontally with the other at the same
time.

3. In learning every kind of science we voluntarily associate many tribes
and trains of ideas, which afterwards are ready for all the purposes either