tion, sensation, volition, or association; that is, to
express the activity or motion of the spirit of animation, at the time it
produces the contractions of the fibrous parts of the system. It may be
supposed that there may exist a greater or less mobility of the fibrous
parts of our system, or a propensity to be stimulated into contraction by
the greater or less quantity or energy of the spirit of animation; and that
hence if the exertion of the sensorial power be in its natural state, and
the mobility of the fibres be increased, the same quantity of fibrous
contraction will be caused, as if the mobility of the fibres continues in
its natural state, and the sensorial exertion be increased.

Thus it may be conceived, that in diseases accompanied with strength, as in
inflammatory fevers with arterial strength, that the cause of greater
fibrous contraction, may exist in the increased mobility of the fibres,
whose contractions are thence both more forceable and more frequent. And
that in diseases attended with debility, as in nervous fevers, where the
fibrous contractions are weaker, and more frequent, it may be conceived
that the cause consists in a decrease of mobility of the fibres; and that
those weak constitutions, which are attended with cold extremities and
large pupils of the eyes, may possess less mobility of the contractile
fibres, as well as less quantity of exertion of the spirit of animation.

In answer to this mode of reasoning it may be sufficient to observe, that
the contractile fibres consist of inert matter, and when the sensorial
power is withdrawn, as in death, they possess no power of motion at all,
but remain in their last state, whether of contraction or relaxation, and
must thence derive the whole of this property from the spirit of animation.
At the same time it is not improbable, that the moving fibres of strong
people may possess a capability of receiving or containing a greater
quantity of the spirit of animation than those of weak people.

In every contraction of a fibre there is an expenditure of the sensorial
power, or spirit of animation; and where the exertion of this sensorial
power has been for some time increased, and the muscles or organs of sense
have in consequence acted with greater energy, its propensity to activity
is proportionally lessened; which is to be ascribed to the exhaustion or
diminution of its quantity. On the contrary, where there has been less
fibrous contraction than