Zoonomia, Vol. I | Page 7

mechanical proportion to
their cause; for the goad of a spur on the skin of a horse shall induce
him to move a load of hay. They differ from the gravitating motions
there mentioned as they are exerted with equal facility in all directions,
and they differ from the chemical class of motions, because no apparent
decompositions or new combinations are produced in the moving
materials.
Hence, when we say animal motion is excited by irritation, we do not
mean that the motion bears any proportion to the mechanical impulse
of the stimulus; nor that it is affected by the general gravitation of the
two bodies; nor by their chemical properties, but solely that certain
animal fibres are excited into action by something external to the
moving organ.
In this sense the stimulus of the blood produces the contractions of the
heart; and the substances we take into our stomach and bowels

stimulate them to perform their necessary functions. The rays of light
excite the retina into animal motion by their stimulus; at the same time
that those rays of light themselves are physically converged to a focus
by the inactive humours of the eye. The vibrations of the air stimulate
the auditory nerve into animal action; while it is probable that the
tympanum of the ear at the same time undergoes a mechanical
vibration.
To render this circumstance more easy to be comprehended, _motion
may be defined to be a variation of figure_; for the whole universe may
be considered as one thing possessing a certain figure; the motions of
any of its parts are a variation of this figure of the whole: this definition
of motion will be further explained in Section XIV. 2. 2. on the
production of ideas.
Now the motions of an organ of sense are a succession of
configurations of that organ; these configurations succeed each other
quicker or slower; and whatever configuration of this organ of sense,
that is, whatever portion of the motion of it is, or has usually been,
attended to, constitutes an idea. Hence the configuration is not to be
considered as an effect of the motion of the organ, but rather as a part
or temporary termination of it; and that, whether a pause succeeds it, or
a new configuration immediately takes place. Thus when a succession
of moving objects are presented to our view, the ideas of trumpets,
horns, lords and ladies, trains and canopies, are configurations, that is,
parts or links of the successive motions of the organ of vision.
[Illustration: Plate I.]
These motions or configurations of the organs of sense differ from the
sensorial motions to be described hereafter, as they appear to be simply
contractions of the fibrous extremities of those organs, and in that
respect exactly resemble the motions or contractions of the larger
muscles, as appears from the following experiment. Place a circular
piece of red silk about an inch in diameter on a sheet of white paper in
a strong light, as in Plate I.--look for a minute on this area, or till the
eye becomes somewhat fatigued, and then, gently closing your eyes,
and shading them with your hand, a circular green area of the same

apparent diameter becomes visible in the closed eye. This green area is
the colour reverse to the red area, which had been previously inspected,
as explained in the experiments on ocular spectra at the end of the work,
and in Botanical Garden, P. 1. additional note, No. 1. Hence it appears,
that a part of the retina, which had been fatigued by contraction in one
direction, relieves itself by exerting the antagonist fibres, and
producing a contraction in an opposite direction, as is common in the
exertions of our muscles. Thus when we are tired with long action of
our arms in one direction, as in holding a bridle on a journey, we
occasionally throw them into an opposite position to relieve the
fatigued muscles.
Mr. Locke has defined an idea to be "whatever is present to the mind;"
but this would include the exertions of volition, and the sensations of
pleasure and pain, as well as those operations of our system, which
acquaint us with external objects; and is therefore too unlimited for our
purpose. Mr. Lock seems to have fallen into a further error, by
conceiving, that the mind could form a general or abstract idea by its
own operation, which was the copy of no particular perception; as of a
triangle in general, that was neither acute, obtuse, nor right angled. The
ingenious Dr. Berkley and Mr. Hume have demonstrated, that such
general ideas have no existence in nature, not even in the mind of their
celebrated inventor. We shall therefore take for granted at present, that
our recollection or imagination of external