too frequently, gradually
lose their effect; as wine and opium cease to intoxicate: some
disagreeable tastes as tobacco, by frequent repetition cease to be
disagreeable; grief and pain gradually diminish and at length cease
altogether; and hence life itself becomes tolerable.

This diminished power of contraction of the fibres of the muscles or
organs of sense, which constitutes permanent debility or old age, may
arise from a deficient secretion of sensorial power in the brain, as
well as from the disobedience of the muscles and organs of sense to
their usual stimuli; but this less production of sensorial power must
depend on the inactivity of the glands, which compose the brain, and
are believed to separate it perpetually from the blood; and is thence
owing to a similar cause with the inaction of the fibres of the other
parts of the system.

It is finally easy to understand how the fibres may cease to act by
the usual quantity of stimulus after having been previously exposed to
a greater quantity of stimulus, or to one too long continued; because
the expenditure of sensorial power has then been greater than its
production; but it is not easy to explain why the repetition of
fibrous contractions, which during the meridian of life did not expend
the sensorial power faster than it was produced; or only in such a
degree as was daily restored by rest and sleep, should at length in
the advance of life expend too much of it; or otherwise, that less of
it should be produced in the brain; or reside in the nerves; lastly
that the fibres should become less excitable by the usual quantity of
it.

5. But these facts would seem to show, that all parts of the system
are not changed as we advance in life, as some have supposed; as in
that case it might have preserved for ever its excitability; and it
might then perhaps have been easier for nature to have continued her
animals and vegetables for ever in their mature state, than
perpetually by a complicate apparatus to have produced new ones, and
suffer the old ones to perish; for a further account of stimulus and
the consequent animal exertion, see Zoonomia, Vol. I. Sect. 12.

II. _Means of preventing old age._

The means of preventing the approach of age must therefore consist in
preventing the inexcitability of the fibres, or the diminution of the
production of sensorial power.

1. As animal motion cannot be performed without the fluid matter of
heat, in which all things ar