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
object last beheld compared with the present one. The same occurs to our
sense of heat, and to every part of our system, which is capable of being
excited into action.

When this variation of the exertion of the sensorial power becomes much and
permanently above or beneath the natural quantity, it becomes a disease. If
the irritative motions be too great or too little, it shews that the
stimulus of external things affect this sensorial power too violently or
too inertly. If the sensitive motions be too great or too little, the cause
arises from the deficient or exuberant quantity of sensation produced in
consequence of the motions of the muscular fibres or organs of sense; if
the voluntary actions are diseased the cause is to be looked for in the
quantity of volition produced in consequence of the desire or aversion
occasioned by the painful or pleasurable sensations above mentioned. And
the diseases of associations probably depend on the greater or less
quantity of the other three sensorial powers by which they were formed.

From whence it appears that the propensity to action, whether it be called
irritability, sensibility, voluntarity, or associability, is only another
mode of expression for the quantity of sensorial power residing in the
organ to be excited. And that on the contrary the words inirritability and
insensibility, together with inaptitude to voluntary and associate motions,
are synonymous with deficiency of the quantity of sensorial power, or of
the spirit of animation, residing in the organs to be excited.

II. _Of sensorial Exertion._

1. There are three circumstances to be attended to in the production of
animal motions, 1st. The stimulus. 2d. The sensorial power. 3d. The
contractile fibre. 1st. A stimulus, external to the organ, originally
induces into action the sensorial faculty termed irritation; this produces
the contraction of the fibres, which, if it be perceived at all, introduces
pleasure or pain; which in their active state are termed sensation; which
is another sensorial fa