s and
sensorial motions reciprocally introduce each other in progressive trains
or tribes, it is termed _catenation_ of animal motions. All these
connections are said to be produced by _habit_; that is, by frequent
repetition.

12. It may be proper to observe, that by the unavoidable idiom of our
language the ideas of perception, of recollection, or of imagination, in
the plural number signify the ideas belonging to perception, to
recollection, or to imagination; whilst the idea of perception, of
recollection, or of imagination, in the singular number is used for what is
termed "a reflex idea of any of those operations of the sensorium."

13. By the word _stimulus_ is not only meant the application of external
bodies to our organs of sense and muscular fibres, which excites into
action the sensorial power termed irritation; but also pleasure or pain,
when they excite into action the sensorial power termed sensation; and
desire or aversion, when they excite into action the power of volition; and
lastly, the fibrous contractions which precede association; as is further
explained in Sect. XII. 2. 1.

* * * * *

SECT. III.

THE MOTIONS OF THE RETINA DEMONSTRATED BY EXPERIMENTS.

I. _Of animal motions and of ideas._ II. _The fibrous structure of the
retina._ III. _The activity of the retina in vision._ 1. _Rays of light
have no momentum._ 2. _Objects long viewed become fainter._ 3. _Spectra
of black objects become luminous._ 4. _Varying spectra from gyration._
5. _From long inspection of various colours._ IV. _Motions of the
organs of sense constitute ideas._ 1. _Light from pressing the
eye-ball, and sound from the pulsation of the carotid artery._ 2.
_Ideas in sleep mistaken for perceptions._ 3. _Ideas of imagination
produce pain and sickness like sensations._ 4. _When the organ of sense
is destroyed, the ideas belonging to that sense perish._ V. _Analogy
between muscular motions and sensual motions, or ideas._ 1. _They are
both originally excited by irritations._ 2. _And associated together in
the same manner._ 3. _Both act in nearly the same times._ 4. _Are alike
strengthened or fatigued by exercise._ 5. _Are alike painful from
inflammation._ 6. _Are alike benumbed by compression._ 7. _Are alike
liable to paralysis._ 8. _To convulsion._ 9. _To the influence of old
age._--VI. _Objections answered._ 1. _Why we cannot inv