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power is withdrawn, as in death, they possess no power of motion at all,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 usual for a certain time, the sensorial power or
spirit of animation becomes accumulated in the inactive part of the system.
Hence vigour succeeds rest, and hence the propensity to action of all our
organs of sense and muscles is in a state of perpetual fluctuation. The
irritability for instance of the retina, that is, its quantity of sensorial
power, varies every moment according to the brightness or obscurity of the