unless its excess is great indeed) and thence to
give permanent strength and energy to the system; I mean that of
volition. This appears not only from the temporary strength of angry
or insane people, but because insanity even cures some diseases of
debility, as I have seen in dropsy, and in some fevers; but it is also
observable, that many who have exerted much voluntary effort during
their whole lives, have continued active to great age. This however
may be conceived to arise from these great exertions being performed
principally by the organs of sense, that is by exciting and comparing
ideas; as in those who have invented sciences, or have governed
nations, and which did not therefore exhaust the sensorial power of
those organs which are necessary to life, but perhaps rather prevented
them from being sooner impaired, their sensorial power not having been
so frequently exhausted by great activity, for very violent exercise
of the body, long continued, forwards old age; as is seen in
post-horses that are cruelly treated, and in many of the poor, who
with difficulty support their families by incessant labour.

III. _Theory of the Approach of Age._

The critical reader is perhaps by this time become so far interested
in this subject as to excuse a more prolix elucidation of it.

In early life the repetition of animal actions occasions them to be
performed with greater facility, whether those repetitions are
produced by volition, sensation, or irritation; because they soon
become associated together, if as much sensorial power is produced
between every reiteration of action, as is expended by it.

But if a stimulus be repeated at uniform intervals of time, the
action, whether of our muscles or organs of sense, is performed with
still greater facility and energy; because the sensorial power of
association mentioned above, is combined with the sensorial power of
irritation, and forms part of the diurnal chain of animal motions;
that is, in common language, the acquired habit assists the power of
the stimulus; see Zoonomia, Vol. I. Sect. XXII. 2. and Sect. XII. 3.
3.

On this circumstance depends the easy motions of the fingers in
performing music, and of the feet and arms in dancing and fencing, and
of the hands in the use of tools in mechanic arts, as well as all the
vital motions which animate and nourish organic bodies.

On the contrary, many animal motions by perpetual repetition are
performed with less energy;