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Title: Zoonomia, Vol. I
       Or, the Laws of Organic Life

Author: Erasmus Darwin

Release Date: April 25, 2005 [EBook #15707]

Language: English

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ZOONOMIA;

OR,

THE LAWS

OF

ORGANIC LIFE.

VOL. I.

_By ERASMUS DARWIN, M.D. F.R.S._

AUTHOR OF THE BOTANIC GARDEN.

       *       *       *       *       *

  Principiò coelum, ac terras, camposque liquentes,
  Lucentemque globum lunæ, titaniaque astra,
  Spiritus intùs alit, totamque infusa per artus
  Mens agitat molem, et magno se corpore miscet.--VIRG. Æn. vi.

  Earth, on whose lap a thousand nations tread,
  And Ocean, brooding his prolific bed,
  Night's changeful orb, blue pole, and silvery zones,
  Where other worlds encircle other suns,
  One Mind inhabits, one diffusive Soul
  Wields the large limbs, and mingles with the whole.

       *       *       *       *       *

_THE SECOND EDITION, CORRECTED._

       *       *       *       *       *

LONDON:
PRINTED FOR. J. JOHNSON, IN ST. PAUL'S CHURCH-YARD.
1796.

Entered at Stationers' Hall.

       *       *       *       *       *

DEDICATION.

To the candid and ingenious Members of the College of Physicians, of the
Royal Philosophical Society, of the Two Universities, and to all those, who
study the Operations of the Mind as a Science, or who practice Medicine as
a Profession, the subsequent Work is, with great respect, inscribed by the
Author,

DERBY, May 1, 1794.

CONTENTS.

            _Preface._
  SECT. I.  _Of Motion._
       II.  _Explanations and Definitions._
      III.  _The Motions of the Retina demonstrated by Experiments._
       IV.  _Laws of Animal Causation._
        V.  _Of the four Faculties or Motions of the Sensorium._
       VI.  _Of the four Classes of Fibrous Motions._
      VII.  _Of Irritative Motions._
     VIII.  _Of Sensitive Motions._
       IX.  _Of Voluntary Motions._
        X.  _Of Associate Motions._
       XI.  _Additional Observations on the Sensorial Powers._
      XII.  _Of Stimulus, Sensorial Exertion, and Fibrous Contraction._
     XIII.  _Of Vegetable Animation._
      XIV.  _Of the Production of Ideas._
       XV.  _Of the Classes of Ideas._
      XVI.  _Of Instinct._
     XVII.  _The Catenation of Animal Motions._
    XVIII.  _Of Sleep._
      XIX.  _Of Reverie._
       XX.  _Of Vertigo._
      XXI.  _Of Drunkenness._
     XXII.  _Of Propensity to Motion. Repetition. Imitation._
    XXIII.  _Of the Circulatory System._
     XXIV.  _Of the Secretion of Saliva, and of Tears. And of the
            Lacrymal Sack._
      XXV.  _Of the Stomach and Intestines._
     XXVI.  _Of the Capillary Glands, and of the Membranes._
    XXVII.  _Of Hemorrhages._
   XXVIII.  _The Paralysis of the Lacteals._
     XXIX.  _The Retrograde Motions of the Absorbent Vessels._
      XXX.  _The Paralysis of the Liver._
     XXXI.  _Of Temperaments._
    XXXII.  _Diseases of Irritation._
   XXXIII.  ---- _of Sensation._
    XXXIV.  ---- _of Volition._
     XXXV.  ---- _of Relation._
    XXXVI.  _The Periods of Diseases._
   XXXVII.  _Of Digestion, Secretion, Nutrition._
  XXXVIII.  _Of the Oxygenation of the Blood in the Lungs and Placenta._
    XXXIX.  _Of Generation._
       XL.  _Of Ocular Spectra._

       *       *       *       *       *

TO

ERASMUS DARWIN,

ON HIS WORK INTITLED

ZOONOMIA,

_By DEWHURST BILSBORROW._

       *       *       *       *       *

  HAIL TO THE BARD! who sung, from Chaos hurl'd
  How suns and planets form'd the whirling world;
  How sphere on sphere Earth's hidden strata bend,
  And caves of rock her central fires defend;
  Where gems new-born their twinkling eyes unfold,               5
  And young ores shoot in arborescent gold.
    How the fair Flower, by Zephyr woo'd, unfurls
  Its panting leaves, and waves its azure curls;
  Or spreads in gay undress its lucid form
  To meet the sun, and shuts it to the storm;                   10
  While in green veins impassion'd eddies move,
  And Beauty kindles into life and love.
    How the first embryon-fibre, sphere, or cube,
  Lives in new forms,--a line,--a ring,--a tube;
  Closed in the womb with limbs unfinish'd laves,               15
  Sips with rude mouth the salutary waves;
  Seeks round its cell the sanguine streams, that pass,
  And drinks with crimson gills the vital gas;
  Weaves with soft threads the blue meandering vein,
  The heart's red concave, and the silver brain;                20
  Leads the long nerve, expands the impatient sense,
  And clothes in silken skin the nascent Ens.
    Erewhile, emerging from its liquid bed,
  It lifts in gelid air its nodding head;
  The lights first dawn with trembling eyelid hails,            25
  With lungs untaught arrests the balmy gales;
  Tries its new tongue in tones unknown, and hears
  The strange vibrations with unpractised ears;
  Seeks with spread hands the bosom's velvet orbs.
  With closing lips the milky fount absorbs;                    30
  And, as compress'd the dulcet streams distil,
  Drinks warmth and fragrance from the living rill;--
  Eyes with mute rapture every waving line,
  Prints with adoring kiss the Paphian shrine,
  And learns erelong, the perfect form confess'd,               35
  Ideal Beauty from its mother's breast.
    Now in strong lines, with bolder tints design'd,
  You sketch ideas, and portray the mind;
  Teach how fine atoms of impinging light
  To ceaseless change the visual sense excite;                  40
  While the bright lens collects the rays, that swerve,
  And bends their focus on the moving nerve.
  How thoughts to thoughts are link'd with viewless chains,
  Tribes leading tribes, and trains pursuing trains;
  With shadowy trident how Volition guides,                     45
  Surge after surge, his intellectual tides;
  Or, Queen of Sleep, Imagination roves
  With frantic Sorrows, or delirious Loves.
    Go on, O FRIEND! explore with eagle-eye;
  Where wrapp'd in night retiring Causes lie:                   50
  Trace their slight bands, their secret haunts betray,
  And give new wonders to the beam of day;
  Till, link by link with step aspiring trod,
  You climb from NATURE to the throne of GOD.
  --So saw the Patriarch with admiring eyes                     55
  From earth to heaven a golden ladder rise;
  Involv'd in clouds the mystic scale ascends,
  And brutes and angels crowd the distant ends.

TRIN. COL. CAMBRIDGE, _Jan._ 1, 1794.

       *       *       *       *       *

REFERENCES TO THE WORK.

          _Botanic Garden._ Part I.

  Line 1.  Canto I.  l. 105.
  ---- 3.  ---- IV.  l. 402.
  ---- 4.  ---- I.  l. 140.
  ---- 5.  ---- III.  l. 401.
  ---- 8.  ---- IV.  l. 452.
  ---- 9.  ---- I.  l. 14.


          _Zoonomia._

  ---- 12.  Sect. XIII.
  ---- 13.  ---- XXXIX. 4. 1.
  ---- 18.  ---- XVI. 2. and XXXVIII.
  ---- 26.  ---- XVI. 4.
  ---- 30.  ---- XVI. 4.
  ---- 36.  ---- XVI. 6.
  ---- 38.  ---- III. and VII.
  ---- 43.  ---- X.
  ---- 44.  ---- XVIII. 17.
  ---- 45.  ---- XVII. 3. 7.
  ---- 47.  ---- XVIII. 8.
  ---- 50.  ---- XXXIX. 4. 8.
  ---- 51.  ---- XXXIX the Motto.
  ---- 54.  ---- XXXIX. 8.

       *       *       *       *       *

PREFACE.

       *       *       *       *       *

The purport of the following pages is an endeavour to reduce the facts
belonging to ANIMAL LIFE into classes, orders, genera, and species; and, by
comparing them with each other, to unravel the theory of diseases. It
happened, perhaps unfortunately for the inquirers into the knowledge of
diseases, that other sciences had received improvement previous to their
own; whence, instead of comparing the properties belonging to animated
nature with each other, they, idly ingenious, busied themselves in
attempting to explain the laws of life by those of mechanism and chemistry;
they considered the body as an hydraulic machine, and the fluids as passing
through a series of chemical changes, forgetting that animation was its
essential characteristic.

The great CREATOR of all things has infinitely diversified the works of his
hands, but has at the same time stamped a certain similitude on the
features of nature, that demonstrates to us, that _the whole is one family
of one parent_. On this similitude is founded all rational analogy; which,
so long as it is concerned in comparing the essential properties of bodies,
leads us to many and important discoveries; but when with licentious
activity it links together objects, otherwise discordant, by some fanciful
similitude; it may indeed collect ornaments for wit and poetry, but
philosophy and truth recoil from its combinations.

The want of a theory, deduced from such strict analogy, to conduct the
practice of medicine is lamented by its professors; for, as a great number
of unconnected facts are difficult to be acquired, and to be reasoned from,
the art of medicine is in many instances less efficacious under the
direction of its wisest practitioners; and by that busy crowd, who either
boldly wade in darkness, or are led into endless error by the glare of
false theory, it is daily practised to the destruction of thousands; add to
this the unceasing injury which accrues to the public by the perpetual
advertisements of pretended nostrums; the minds of the indolent become
superstitiously fearful of diseases, which they do not labour under; and
thus become the daily prey of some crafty empyric.

A theory founded upon nature, that should bind together the scattered facts
of medical knowledge, and converge into one point of view the laws of
organic life, would thus on many accounts contribute to the interest of
society. It would capacitate men of moderate abilities to practise the art
of healing with real advantage to the public; it would enable every one of
literary acquirements to distinguish the genuine disciples of medicine from
those of boastful effrontery, or of wily address; and would teach mankind
in some important situations the _knowledge of themselves_.

There are some modern practitioners, who declaim against medical theory in
general, not considering that to think is to theorize; and that no one can
direct a method of cure to a person labouring under disease without
thinking, that is, without theorizing; and happy therefore is the patient,
whose physician possesses the best theory.

The words idea, perception, sensation, recollection, suggestion, and
association, are each of them used in this treatise in a more limited sense
than in the writers of metaphysic. The author was in doubt, whether he
should rather have substituted new words instead of them; but was at length
of opinion, that new definitions of words already in use would be less
burthensome to the memory of the reader.

A great part of this work has lain by the writer above twenty years, as
some of his friends can testify: he had hoped by frequent revision to have
made it more worthy the acceptance of the public; this however his other
perpetual occupations have in part prevented, and may continue to prevent,
as long as he may be capable of revising it; he therefore begs of the
candid reader to accept of it in its present state, and to excuse any
inaccuracies of expression, or of conclusion, into which the intricacy of
his subject, the general imperfection of language, or the frailty he has in
common with other men, may have betrayed him; and from which he has not the
vanity to believe this treatise to be exempt.

       *       *       *       *       *

ZOONOMIA.

       *       *       *       *       *

SECT. I.

OF MOTION.

The whole of nature may be supposed to consist of two essences or
substances; one of which may be termed spirit, and the other matter. The
former of these possesses the power to commence or produce motion, and the
latter to receive and communicate it. So that motion, considered as a
cause, immediately precedes every effect; and, considered as an effect, it
immediately succeeds every cause.

The MOTIONS OF MATTER may be divided into two kinds, primary and secondary.
The secondary motions are those, which are given to or received from other
matter in motion. Their laws have been successfully investigated by
philosophers in their treatises on mechanic powers. These motions are
distinguished by this circumstance, that the velocity multiplied into the
quantity of matter of the body acted upon is equal to the velocity
multiplied into the quantity of matter of the acting body.

The primary motions of matter may be divided into three classes, those
belonging to gravitation, to chemistry, and to life; and each class has its
peculiar laws. Though these three classes include the motions of solid,
liquid, and aerial bodies; there is nevertheless a fourth division of
motions; I mean those of the supposed ethereal fluids of magnetism,
electricity, heat, and light; whose properties are not so well investigated
as to be classed with sufficient accuracy.

_1st._ The gravitating motions include the annual and diurnal rotation of
the earth and planets, the flux and reflux of the ocean, the descent of
heavy bodies, and other phænomena of gravitation. The unparalleled sagacity
of the great NEWTON has deduced the laws of this class of motions from the
simple principle of the general attraction of matter. These motions are
distinguished by their tendency to or from the centers of the sun or
planets.

_2d._ The chemical class of motions includes all the various appearances of
chemistry. Many of the facts, which belong to these branches of science,
are nicely ascertained, and elegantly classed; but their laws have not yet
been developed from such simple principles as those above-mentioned; though
it is probable, that they depend on the specific attractions belonging to
the particles of bodies, or to the difference of the quantity of attraction
belonging to the sides and angles of those particles. The chemical motions
are distinguished by their being generally attended with an evident
decomposition or new combination of the active materials.

_3d._ The third class includes all the motions of the animal and vegetable
world; as well those of the vessels, which circulate their juices, and of
the muscles, which perform their locomotion, as those of the organs of
sense, which constitute their ideas.

This last class of motion is the subject of the following pages; which,
though conscious of their many imperfections, I hope may give some pleasure
to the patient reader, and contribute something to the knowledge and to the
cure of diseases.

       *       *       *       *       *

SECT. II.

EXPLANATIONS AND DEFINITIONS.

    I. _Outline of the animal economy._--II. 1. _Of the sensorium._ 2. _Of
    the brain and nervous medulla._ 3. _A nerve._ 4. _A muscular fibre._ 5.
    _The immediate organs of sense._ 6. _The external organs of sense._ 7.
    _An idea or sensual motion._ 8. _Perception._ 9. _Sensation._ 10.
    _Recollection and suggestion._ 11. _Habit, causation, association,
    catenation._ 12. _Reflex ideas._ 13. _Stimulus defined._

       *       *       *       *       *

    As some explanations and definitions will be necessary in the
    prosecution of the work, the reader is troubled with them in this
    place, and is intreated to keep them in his mind as he proceeds, and to
    take them for granted, till an apt opportunity occurs to evince their
    truth; to which I shall premise a very short outline of the animal
    economy.

       *       *       *       *       *

I.--1. The nervous system has its origin from the brain, and is distributed
to every part of the body. Those nerves, which serve the senses,
principally arise from that part of the brain, which is lodged in the head;
and those, which serve the purposes of muscular motion, principally arise
from that part of the brain, which is lodged in the neck and back, and
which is erroneously called the spinal marrow. The ultimate fibrils of
these nerves terminate in the immediate organs of sense and muscular
fibres, and if a ligature be put on any part of their passage from the head
or spine, all motion and perception cease in the parts beneath the
ligature.

2. The longitudinal muscular fibres compose the locomotive muscles, whose
contractions move the bones of the limbs and trunk, to which their
extremities are attached. The annular or spiral muscular fibres compose the
vascular muscles, which constitute the intestinal canal, the arteries,
veins, glands, and absorbent vessels.

3. The immediate organs of sense, as the retina of the eye, probably
consist of moving fibrils, with a power of contraction similar to that of
the larger muscles above described.

4. The cellular membrane consists of cells, which resemble those of a
sponge, communicating with each other, and connecting together all the
other parts of the body.

5. The arterial system consists of the aortal and the pulmonary artery,
which are attended through their whole course with their correspondent
veins. The pulmonary artery receives the blood from the right chamber of
the heart, and carries it to the minute extensive ramifications of the
lungs, where it is exposed to the action of the air on a surface equal to
that of the whole external skin, through the thin moist coats of those
vessels, which are spread on the air-cells, which constitute the minute
terminal ramifications of the wind-pipe. Here the blood changes its colour
from a dark red to a bright scarlet. It is then collected by the branches
of the pulmonary vein, and conveyed to the left chamber of the heart.

6. The aorta is another large artery, which receives the blood from the
left chamber of the heart, after it has been thus aerated in the lungs, and
conveys it by ascending and descending branches to every other part of the
system; the extremities of this artery terminate either in glands, as the
salivary glands, lacrymal glands, &c. or in capillary vessels, which are
probably less involuted glands; in these some fluid, as saliva, tears,
perspiration, are separated from the blood; and the remainder of the blood
is absorbed or drank up by branches of veins correspondent to the branches
of the artery; which are furnished with valves to prevent its return; and
is thus carried back, after having again changed its colour to a dark red,
to the right chamber of the heart. The circulation of the blood in the
liver differs from this general system; for the veins which drink up the
refluent blood from those arteries, which are spread on the bowels and
mesentery, unite into a trunk in the liver, and form a kind of artery,
which is branched into the whole substance of the liver, and is called the
vena portarum; and from which the bile is separated by the numerous hepatic
glands, which constitute that viscus.

7. The glands may be divided into three systems, the convoluted glands,
such as those above described, which separate bile, tears, saliva, &c.
Secondly, the glands without convolution, as the capillary vessels, which
unite the terminations of the arteries and veins; and separate both the
mucus, which lubricates the cellular membrane, and the perspirable matter,
which preserves the skin moist and flexible. And thirdly, the whole
absorbent system, consisting of the lacteals, which open their mouths into
the stomach and intestines, and of the lymphatics, which open their mouths
on the external surface of the body, and on the internal linings of all the
cells of the cellular membrane, and other cavities of the body.

These lacteal and lymphatic vessels are furnished with numerous valves to
prevent the return of the fluids, which they absorb, and terminate in
glands, called lymphatic glands, and may hence be considered as long necks
or mouths belonging to these glands. To these they convey the chyle and
mucus, with a part of the perspirable matter, and atmospheric moisture; all
which, after having passed through these glands, and having suffered some
change in them, are carried forward into the blood, and supply perpetual
nourishment to the system, or replace its hourly waste.

8. The stomach and intestinal canal have a constant vermicular motion,
which carries forwards their contents, after the lacteals have drank up the
chyle from them; and which is excited into action by the stimulus of the
aliment we swallow, but which becomes occasionally inverted or retrograde,
as in vomiting, and in the iliac passion.

II. 1. The word _sensorium_ in the following pages is designed to express
not only the medullary part of the brain, spinal marrow, nerves, organs of
sense, and of the muscles; but also at the same time that living principle,
or spirit of animation, which resides throughout the body, without being
cognizable to our senses, except by its effects. The changes which
occasionally take place in the sensorium, as during the exertions of
volition, or the sensations of pleasure or pain, are termed _sensorial
motions_.

2. The similarity of the texture of the brain to that of the pancreas, and
some other glands of the body, has induced the inquirers into this subject
to believe, that a fluid, perhaps much more subtile than the electric aura,
is separated from the blood by that organ for the purposes of motion and
sensation. When we recollect, that the electric fluid itself is actually
accumulated and given out voluntarily by the torpedo and the gymnotus
electricus, that an electric shock will frequently stimulate into motion a
paralytic limb, and lastly that it needs no perceptible tubes to convey it,
this opinion seems not without probability; and the singular figure of the
brain and nervous system seems well adapted to distribute it over every
part of the body.

For the medullary substance of the brain not only occupies the cavities of
the head and spine, but passes along the innumerable ramifications of the
nerves to the various muscles and organs of sense. In these it lays aside
its coverings, and is intermixed with the slender fibres, which constitute
those muscles and organs of sense. Thus all these distant ramifications of
the sensorium are united at one of their extremities, that is, in the head
and spine; and thus these central parts of the sensorium constitute a
communication between all the organs of sense and muscles.

3. A _nerve_ is a continuation of the medullary substance of the brain from
the head or spine towards the other parts of the body, wrapped in its
proper membrane.

4. The _muscular fibres_ are moving organs intermixed with that medullary
substance, which is continued along the nerves, as mentioned above. They
are indued with the power of contraction, and are again elongated either by
antagonist muscles, by circulating fluids, or by elastic ligaments. So the
muscles on one side of the forearm bend the fingers by means of their
tendons, and those on the other side of the fore-arm extend them again. The
arteries are distended by the circulating blood; and in the necks of
quadrupeds there is a strong elastic ligament, which assists the muscles,
which elevate the head, to keep it in its horizontal position, and to raise
it after it has been depressed.

5. The _immediate organs of sense_ consist in like manner of moving fibres
enveloped in the medullary substance above mentioned; and are erroneously
supposed to be simply an expansion of the nervous medulla, as the retina of
the eye, and the rete mucosum of the skin, which are the immediate organs
of vision, and of touch. Hence when we speak of the contractions of the
fibrous parts of the body, we shall mean both the contractions of the
muscles, and those of the immediate organs of sense. These _fibrous
motions_ are thus distinguished from the _sensorial motions_ above
mentioned.

6. The _external organs_ of sense are the coverings of the immediate organs
of sense, and are mechanically adapted for the reception or transmission of
peculiar bodies, or of their qualities, as the cornea and humours of the
eye, the tympanum of the ear, the cuticle of the finders and tongue.

7. The word _idea_ has various meanings in the writers of metaphysic: it is
here used simply for those notions of external things, which our organs of
sense bring us acquainted with originally; and is defined a contraction, or
motion, or configuration, of the fibres, which constitute the immediate
organ of sense; which will be explained at large in another part of the
work. Synonymous with the word idea, we shall sometimes use the words
_sensual motion_ in contradistinction to _muscular motion_.

8. The word _perception_ includes both the action of the organ of sense in
consequence of the impact of external objects, and our attention to that
action; that is, it expresses both the motion of the organ of sense, or
idea, and the pain or pleasure that succeeds or accompanies it.

9. The pleasure or pain which necessarily accompanies all those perceptions
or ideas which we attend to, either gradually subsides, or is succeeded by
other fibrous motions. In the latter case it is termed _sensation_, as
explained in Sect. V. 2, and VI. 2.--The reader is intreated to keep this
in his mind, that through all this treatise the word sensation is used to
express pleasure or pain only in its active state, by whatever means it is
introduced into the system, without any reference to the stimulation of
external objects.

10. The vulgar use of the word _memory_ is too unlimited for our purpose:
those ideas which we voluntarily recall are here termed ideas of
_recollection_, as when we will to repeat the alphabet backwards. And those
ideas which are suggested to us by preceding ideas are here termed ideas of
_suggestion_, as whilst we repeat the alphabet in the usual order; when by
habits previously acquired B is suggested by A, and C by B, without any
effort of deliberation.

11. The word _association_ properly signifies a society or convention of
things in some respects similar to each other. We never say in common
language, that the effect is associated with the cause, though they
necessarily accompany or succeed each other. Thus the contractions of our
muscles and organs of sense may be said to be associated together, but
cannot with propriety be said to be associated with irritations, or with
volition, or with sensation; because they are caused by them, as mentioned
in Sect. IV. When fibrous contractions succeed other fibrous contractions,
the connection is termed _association_; when fibrous contractions succeed
sensorial motions, the connection is termed _causation_; when fibrous 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 invent new ideas._
    2. _If ideas resemble external objects._ 3. _Of the imagined sensation
    in an amputated limb._ 4. _Abstract ideas._--VII. _What are ideas, if
    they are not animal motions?_

Before the great variety of animal motions can be duly arranged into
natural classes and orders, it is necessary to smooth the way to this yet
unconquered field of science, by removing some obstacles which thwart our
passage. I. To demonstrate that the retina and other immediate organs of
sense possess a power of motion, and that these motions constitute our
ideas, according to the fifth and seventh of the preceding assertions,
claims our first attention.

Animal motions are distinguished from the communicated motions, mentioned
in the first section, as they have no 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 objects consists of a partial repetition of the
perceptions, which were excited by those external objects, at the time we
became acquainted with them; and that our reflex ideas of the operations of
our minds are partial repetitions of those operations.

II. The following article evinces that the organ of vision consists of a
fibrous part as well as of the nervous medulla, like other white muscles;
and hence, as it resembles the muscular parts of the body in its structure,
we may conclude, that it must resemble them in possessing a power of being
excited into animal motion.--The subsequent experiments on the optic nerve,
and on the colours remaining in the eye, are copied from a paper on ocular
spectra published in the seventy-sixth volume of the Philos. Trans. by Dr.
R. Darwin of Shrewsbury; which, as I shall have frequent occasion to refer
to, is reprinted in this work, Sect. XL. The retina of an ox's eye was
suspended in a glass of warm water, and forcibly torn in a few places; the
edges of these parts appeared jagged and hairy, and did not contract and
become smooth like simple mucus, when it is distended till it breaks; which
evinced that it consisted of fibres. This fibrous construction became still
more distinct to the light by adding some caustic alcali to the water; as
the adhering mucus was first eroded, and the hair-like fibres remained
floating in the vessel. Nor does the degree of transparency of the retina
invalidate this evidence of its fibrous structure, since Leeuwenhoek has
shewn, that the crystalline humour itself consists of fibres. Arc. Nat. V.
I. 70.

Hence it appears, that as the muscles consist of larger fibres intermixed
with a smaller quantity of nervous medulla, the organ of vision consists of
a greater quantity of nervous medulla intermixed with smaller fibres. It is
probable that the locomotive muscles of microscopic animals may have
greater tenuity than these of the retina; and there is reason to conclude
from analogy, that the other immediate organs of sense, as the portio
mollis of the auditory nerve, and the rete mucosum of the skin, possess a
similarity of structure with the retina, and a similar power of being
excited into animal motion.

III. The subsequent articles shew, that neither mechanical impressions, nor
chemical combinations of light, but that the animal activity of the retina
constitutes vision.

1. Much has been conjectured by philosophers about the momentum of the rays
of light; to subject this to experiment a very light horizontal balance was
constructed by Mr. Michel, with about an inch square of thin leaf-copper
suspended at each end of it, as described in Dr. Priestley's History of
Light and Colours. The focus of a very large convex mirror was thrown by
Dr. Powel, in his lectures on experimental philosophy, in my presence, on
one wing of this delicate balance, and it receded from the light; thrown on
the other wing, it approached towards the light, and this repeatedly; so
that no sensible impulse could be observed, but what might well be ascribed
to the ascent of heated air.

Whence it is reasonable to conclude, that the light of the day must be much
too weak in its dilute state to make any mechanical impression on so
tenacious a substance as the retina of the eye.--Add to this, that as the
retina is nearly transparent, it could therefore make less resistance to
the mechanical impulse of light; which, according, to the observations
related by Mr. Melvil in the Edinburgh Literary Essays, only communicates
heat, and should therefore only communicate momentum, where it is
obstructed, reflected, or refracted.--From whence also may be collected the
final cause of this degree of transparency of the retina, viz. left by the
focus of stronger lights, heat and pain should have been produced in the
retina, instead of that stimulus which excites it into animal motion.

2. On looking long on an area of scarlet silk of about an inch in diameter
laid on white paper, as in Plate I. the scarlet colour becomes fainter,
till at length it entirely vanishes, though the eye is kept uniformly and
steadily upon it. Now if the change or motion of the retina was a
mechanical impression, or a chemical tinge of coloured light, the
perception would every minute become stronger and stronger,--whereas in
this experiment it becomes every instant weaker and weaker. The same
circumstance obtains in the continued application of sound, or of sapid
bodies, or of odorous ones, or of tangible ones, to their adapted organs of
sense.

[Illustration: Plate II.]

Thus when a circular coin, as a shilling, is pressed on the palm of the
hand, the sense of touch is mechanically compressed; but it is the stimulus
of this pressure that excites the organ of touch into animal action, which
constitutes the perception of hardness and of figure; for in some minutes
the perception ceases, though the mechanical pressure of the object
remains.

3. Make with ink on white paper a very black spot about half an inch in
diameter, with a tail about an inch in length, so as to resemble a tadpole,
as in Plate II.; look steadfastly for a minute on the center of this spot,
and, on moving the eye a little, the figure of the tadpole will be seen on
the white part of the paper; which figure of the tadpole will appear more
luminous than the other part of the white paper; which can only be
explained by supposing that a part of the retina, on which the tadpole was
delineated, to have become more sensible to light than the other parts of
it, which were exposed to the white paper; and not from any idea of
mechanical impression or chemical combination of light with the retina.

4. When any one turns round rapidly, till he becomes dizzy, and falls upon
the ground, the spectra of the ambient objects continue to present
themselves in rotation, and he seems to behold the objects still in motion.
Now if these spectra were impressions on a passive organ, they either must
continue as they were received last, or not continue at all.

5. Place a piece of red silk about an inch in diameter on a sheet of white
paper in a strong light, as in Plate I; look steadily upon it from the
distance of about half a yard for a minute; then closing your eye-lids,
cover them with your hands and handkerchief, and a green spectrum will be
seen in your eyes resembling in form the piece of red silk. After some
seconds of time the spectrum will disappear, and in a few more seconds will
reappear; and thus alternately three or four times, if the experiment be
well made, till at length it vanishes entirely.

[Illustration: Plate III.]

6. Place a circular piece of white paper, about four inches in diameter, in
the sunshine, cover the center of this with a circular piece of black silk,
about three inches in diameter; and the center of the black silk with a
circle of pink silk, about two inches in diameter; and the center of the
pink silk with a circle of yellow silk, about one inch in diameter; and the
center of this with a circle of blue silk, about half an inch in diameter;
make a small spot with ink in the center of the blue silk, as in Plate
III.; look steadily for a minute on this central spot, and then closing
your eyes, and applying your hand at about an inch distance before them, so
as to prevent too much or too little light from passing through the
eye-lids, and you will see the most beautiful circles of colours that
imagination can conceive; which are most resembled by the colours
occasioned by pouring a drop or two of oil on a still lake in a bright day.
But these circular irises of colours are not only different from the
colours of the silks above mentioned, but are at the same time perpetually
changing as long as they exist.

From all these experiments it appears, that these spectra in the eye are
not owing to the mechanical impulse of light impressed on the retina; nor
to its chemical combination with that organ; nor to the absorption and
emission of light, as is supposed, perhaps erroneously, to take place in
calcined shells and other phosphorescent bodies, after having been exposed
to the light: for in all these cases the spectra in the eye should either
remain of the same colour, or gradually decay, when the object is
withdrawn; and neither their evanescence during the presence of their
object, as in the second experiment, nor their change from dark to
luminous, as in the third experiment, nor their rotation, as in the fourth
experiment, nor the alternate presence and evanescence of them, as in the
fifth experiment, nor the perpetual change of colours of them, as in the
last experiment, could exist.

IV. The subsequent articles shew, that these animal motions or
configurations of our organs of sense constitute our ideas.

1. If any one in the dark presses the ball of his eye, by applying his
finger to the external corner of it, a luminous appearance is observed; and
by a smart stroke on the eye great slashes of fire are perceived. (Newton's
Optics.) So that when the arteries, that are near the auditory nerve, make
stronger pulsations than usual, as in some fevers, an undulating sound is
excited in the ears. Hence it is not the presence of the light and sound,
but the motions of the organ, that are immediately necessary to constitute
the perception or idea of light and sound.

2. During the time of sleep, or in delirium, the ideas of imagination are
mistaken for the perceptions of external objects; whence it appears, that
these ideas of imagination, are no other than a reiteration of those
motions of the organs of sense, which were originally excited by the
stimulus of external objects: and in our waking hours the simple ideas,
that we call up by recollection or by imagination, as the colour of red, or
the smell of a rose, are exact resemblances of the same simple ideas from
perception; and in consequence must be a repetition of those very motions.

3. The disagreeable sensation called the tooth-edge is originally excited
by the painful jarring of the teeth in biting the edge of the glass, or
porcelain cup, in which our food was given us in our infancy, as is further
explained in the Section XVI. 10, on Instinct.--This disagreeable sensation
is afterwards excitable not only by a repetition of the sound, that was
then produced, but by imagination alone, as I have myself frequently
experienced; in this case the idea of biting a china cup, when I imagine it
very distinctly, or when I see another person bite a cup or glass, excites
an actual pain in the nerves of my teeth. So that this idea and pain seem
to be nothing more than the reiterated motions of those nerves, that were
formerly so disagreeably affected.

Other ideas that are excited by imagination or recollection in many
instances produce similar effects on the constitution, as our perceptions
had formerly produced, and are therefore undoubtedly a repetition of the
same motions. A story which the celebrated Baron Van Swieton relates of
himself is to this purpose. He was present when the putrid carcase of a
dead dog exploded with prodigious stench; and some years afterwards,
accidentally riding along the same road, he was thrown into the same
sickness and vomiting by the idea of the stench, as he had before
experienced from the perception of it.

4. Where the organ of sense is totally destroyed, the ideas which were
received by that organ seem to perish along with it, as well as the power
of perception. Of this a satisfactory instance has fallen under my
observation. A gentleman about sixty years of age had been totally deaf for
near thirty years: he appeared to be a man of good understanding, and
amused himself with reading, and by conversing either by the use of the
pen, or by signs made with his fingers, to represent letters. I observed
that he had so far forgot the pronunciation of the language, that when he
attempted to speak, none of his words had distinct articulation, though his
relations could sometimes understand his meaning. But, which is much to the
point, he assured me, that in his dreams he always imagined that people
conversed with him by signs or writing, and never that he heard any one
speak to him. From hence it appears, that with the perceptions of sounds he
has also lost the ideas of them; though the organs of speech still retain
somewhat of their usual habits of articulation.

This observation may throw some light on the medical treatment of deaf
people; as it may be learnt from their dreams whether the auditory nerve be
paralytic, or their deafness be owing to some defect of the external organ.

It rarely happens that the immediate organ of vision is perfectly
destroyed. The most frequent causes of blindness are occasioned by defects
of the external organ, as in cataracts and obfuscations of the cornea. But
I have had the opportunity of conversing with two men, who had been some
years blind; one of them had a complete gutta serena, and the other had
lost the whole substance of his eyes. They both told me that they did not
remember to have ever dreamt of visible objects, since the total loss of
their sight.

V. Another method of discovering that our ideas are animal motions of the
organs of sense, is from considering the great analogy they bear to the
motions of the larger muscles of the body. In the following articles it
will appear that they are originally excited into action by the irritation
of external objects like our muscles; are associated together like our
muscular motions; act in similar time with them; are fatigued by continued
exertion like them; and that the organs of sense are subject to
inflammation, numbness, palsy, convulsion, and the defects of old age, in
the same manner as the muscular fibres.

1. All our perceptions or ideas of external objects are universally allowed
to have been originally excited by the stimulus of those external objects;
and it will be shewn in a succeeding section, that it is probable that all
our muscular motions, as well those that are become voluntary as those of
the heart and glandular system, were originally in like manner excited by
the stimulus of something external to the organ of motion.

2. Our ideas are also associated together after their production precisely
in the same manner as our muscular motions; which will likewise be fully
explained in the succeeding section.

3. The time taken up in performing an idea is likewise much the same as
that taken up in performing a muscular motion. A musician can press the
keys of an harpsichord with his fingers in the order of a tune he has been
accustomed to play, in as little time as he can run over those notes in his
mind. So we many times in an hour cover our eye-balls with our eye-lids
without perceiving that we are in the dark; hence the perception or idea of
light is not changed for that of darkness in so small a time as the
twinkling of an eye; so that in this case the muscular motion of the
eye-lid is performed quicker than the perception of light can be changed
for that of darkness.--So if a fire-stick be whirled round in the dark, a
luminous circle appears to the observer; if it be whirled somewhat slower,
this circle becomes interrupted in one part; and then the time taken up in
such a revolution of the stick is the same that the observer uses in
changing his ideas: thus the [Greek: dolikoskoton enkos] of Homer, the long
shadow of the flying javelin, is elegantly designed to give us an idea of
its velocity, and not of its length.

4. The fatigue that follows a continued attention of the mind to one object
is relieved by changing the subject of our thoughts; as the continued
movement of one limb is relieved by moving another in its stead. Whereas a
due exercise of the faculties of the mind strengthens and improves those
faculties, whether of imagination or recollection; as the exercise of our
limbs in dancing or fencing increases the strength and agility of the
muscles thus employed.

5. If the muscles of any limb are inflamed, they do not move without pain;
so when the retina is inflamed, its motions also are painful. Hence light
is as intolerable in this kind of ophthalmia, as pressure is to the finger
in the paronychia. In this disease the patients frequently dream of having
their eyes painfully dazzled; hence the idea of strong light is painful as
well as the reality. The first of these facts evinces that our perceptions
are motions of the organs of sense; and the latter, that our imaginations
are also motions of the same organs.

6. The organs of sense, like the moving muscles, are liable to become
benumbed, or less sensible, from compression. Thus, if any person on a
light day looks on a white wall, he may perceive the ramifications of the
optic artery, at every pulsation of it, represented by darker branches on
the white wall; which is evidently owing to its compressing the retina
during the diastole of the artery. Savage Nosolog.

7. The organs of sense and the moving muscles are alike liable to be
affected with palsy, as in the gutta serena, and in some cases of deafness;
and one side of the face has sometimes lost its power of sensation, but
retained its power of motion; other parts of the body have lost their
motions but retained their sensation, as in the common hemiplagia; and in
other instances both these powers have perished together.

8. In some convulsive diseases a delirium or insanity supervenes, and the
convulsions cease; and conversely the convulsions shall supervene, and the
delirium cease. Of this I have been a witness many times in a day in the
paroxysms of violent epilepsies; which evinces that one kind of delirium is
a convulsion of the organs of sense, and that our ideas are the motions of
these organs: the subsequent cases will illustrate this observation.

Miss G----, a fair young lady, with light eyes and hair, was seized with
most violent convulsions of her limbs, with outrageous hiccough, and most
vehement efforts to vomit: after near an hour was elapsed this tragedy
ceased, and a calm talkative delirium supervened for about another hour;
and these relieved each other at intervals during the greatest part of
three or four days. After having carefully considered this disease, I
thought the convulsions of her ideas less dangerous than those of her
muscles; and having in vain attempted to make any opiate continue in her
stomach, an ounce of laudanum was rubbed along the spine of her back, and a
dram of it was used as an enema; by this medicine a kind of drunken
delirium was continued many hours; and when it ceased the convulsions did
not return; and the lady continued well many years, except some lighter
relapses, which were relieved in the same manner.

Miss H----, an accomplished young lady, with light eyes and hair, was
seized with convulsions of her limbs, with hiccough, and efforts to vomit,
more violent than words can express; these continued near an hour, and were
succeeded with a cataleptic spasm of one arm, with the hand applied to her
head; and after about twenty minutes these spasms ceased, and a talkative
reverie supervened for near an other hour, from which no violence, which it
was proper to use, could awaken her. These periods of convulsions, first of
the muscles, and then of the ideas, returned twice a day for several weeks;
and were at length removed by great doses of opium, after a great variety
of other medicines and applications had been in vain experienced. This lady
was subject to frequent relapses, once or twice a year for many years, and
was as frequently relieved by the same method.

Miss W----, an elegant young lady, with black eyes and hair, had sometimes
a violent pain of her side, at other times a most painful strangury, which
were every day succeeded by delirium; which gave a temporary relief to the
painful spasms. After the vain exhibition of variety of medicines and
applications by different physicians, for more than a twelvemonth, she was
directed to take some doses of opium, which were gradually increased, by
which a drunken delirium was kept up for a day or two, and the pains
prevented from returning. A flesh diet, with a little wine or beer, instead
of the low regimen she had previously used, in a few weeks completely
established her health; which, except a few relapses, has continued for
many years.

9. Lastly, as we advance in life all the parts of the body become more
rigid, and are rendered less susceptible of new habits of motion, though
they retain those that were before established. This is sensibly observed
by those who apply themselves late in life to music, fencing, or any of the
mechanic arts. In the same manner many elderly people retain the ideas they
had learned early in life, but find great difficulty in acquiring new
trains of memory; insomuch that in extreme old age we frequently see a
forgetfulness of the business of yesterday, and at the same time a
circumstantial remembrance of the amusements of their youth; till at length
the ideas of recollection and activity of the body gradually cease
together,--such is the condition of humanity!--and nothing remains but the
vital motions and sensations.

VI. 1. In opposition to this doctrine of the production of our ideas, it
may be asked, if some of our ideas, like other animal motions, are
voluntary, why can we not invent new ones, that have not been received by
perception? The answer will be better understood after having perused the
succeeding section, where it will be explained, that the muscular motions
likewise are originally excited by the stimulus of bodies external to the
moving organ; and that the will has only the power of repeating the motions
thus excited.

2. Another objector may ask, Can the motion of an organ of sense resemble
an odour or a colour? To which I can only answer, that it has not been
demonstrated that any of our ideas resemble the objects that excite them;
it has generally been believed that they do not; but this shall be
discussed at large in Sect. XIV.

3. There is another objection that at first view would seem less easy to
surmount. After the amputation, of a foot or a finger, it has frequently
happened, that an injury being offered to the stump of the amputated limb,
whether from cold air, too great pressure, or other accidents, the patient
has complained, of a sensation of pain in the foot or finger, that was cut
off. Does not this evince that all our ideas are excited in the brain, and
not in the organs of sense? This objection is answered, by observing that
our ideas of the shape, place, and solidity of our limbs, are acquired by
our organs of touch and of sight, which are situated in our fingers and
eyes, and not by any sensations in the limb itself.

In this case the pain or sensation, which formerly has arisen in the foot
or toes, and been propagated along the nerves to the central part of the
sensorium, was at the same time accompanied with a visible idea of the
shape and place, and with a tangible idea of the solidity of the affected
limb: now when these nerves are afterwards affected by any injury done to
the remaining stump with a similar degree or kind of pain, the ideas of the
shape, place, or solidity of the lost limb, return by association; as these
ideas belong to the organs of sight and touch, on which they were first
excited.

4. If you wonder what organs of sense can be excited into motion, when you
call up the ideas of wisdom or benevolence, which Mr. Locke has termed
abstracted ideas; I ask you by what organs of sense you first became
acquainted with these ideas? And the answer will be reciprocal; for it is
certain that all our ideas were originally acquired by our organs of sense;
for whatever excites our perception must be external to the organ that
perceives it, and we have no other inlets to knowledge but by our
perceptions: as will be further explained in Section XIV. and XV. on the
Productions and Classes of Ideas.

VII. If our recollection or imagination be not a repetition of animal
movements, I ask, in my turn, What is it? You tell me it consists of images
or pictures of things. Where is this extensive canvas hung up? or where are
the numerous receptacles in which those are deposited? or to what else in
the animal system have they any similitude?

That pleasing picture of objects, represented in miniature on the retina of
the eye, seems to have given rise to this illusive oratory! It was forgot
that this representation belongs rather to the laws of light, than to those
of life; and may with equal elegance be seen in the camera obscura as in
the eye; and that the picture vanishes for ever, when the object is
withdrawn.

       *       *       *       *       *

SECT. IV.

LAWS OF ANIMAL CAUSATION.

I. The fibres, which constitute the muscles and organs of sense, possess a
power of contraction. The circumstances attending the exertion of this
power of CONTRACTION constitute the laws of animal motion, as the
circumstances attending the exertion of the power of ATTRACTION constitute
the laws of motion of inanimate matter.

II. The spirit of animation is the immediate cause of the contraction of
animal fibres, it resides in the brain and nerves, and is liable to general
or partial diminution or accumulation.

III. The stimulus of bodies external to the moving organ is the remote
cause of the original contractions of animal fibres.

IV. A certain quantity of stimulus produces irritation, which is an
exertion of the spirit of animation exciting the fibres into contraction.

V. A certain quantity of contraction of animal fibres, if it be perceived
at all, produces pleasure; a greater or less quantity of contraction, if it
be perceived at all, produces pain; these constitute sensation.

VI. A certain quantity of sensation produces desire or aversion; these
constitute volition.

VII. All animal motions which have occurred at the same time, or in
immediate succession, become so connected, that when one of them is
reproduced, the other has a tendency to accompany or succeed it. When
fibrous contractions succeed or accompany other fibrous contractions, the
connection is termed association; when fibrous contractions succeed
sensorial motions, the connexion is termed causation; when fibrous and
sensorial motions reciprocally introduce each other, it is termed
catenation of animal motions. All these connections are said to be produced
by habit, that is, by frequent repetition. These laws of animal causation
will be evinced by numerous facts, which occur in our daily exertions; and
will afterwards be employed to explain the more recondite phænomena of the
production, growth, diseases, and decay of the animal system.

       *       *       *       *       *

SECT. V.

OF THE FOUR FACULTIES OR MOTIONS OF THE SENSORIUM.

    1. _Four sensorial powers._ 2. _Irritation, sensation, volition,
    association defined._ 3. _Sensorial motions distinguished from fibrous
    motions._

1. The spirit of animation has four different modes of action, or in other
words the animal sensorium possesses four different faculties, which are
occasionally exerted, and cause all the contractions of the fibrous parts
of the body. These are the faculty of causing fibrous contractions in
consequence of the irritations excited by external bodies, in consequence
of the sensations of pleasure or pain, in consequence of volition, and in
consequence of the associations of fibrous contractions with other fibrous
contractions, which precede or accompany them.

These four faculties of the sensorium during their inactive state are
termed irritability, sensibility, voluntarity, and associability; in their
active state they are termed as above, irritation, sensation, volition,
association.

2. IRRITATION is an exertion or change of some extreme part of the
sensorium residing in the muscles or organs of sense, in consequence of the
appulses of external bodies.

SENSATION is an exertion or change of the central parts of the sensorium,
or of the whole of it, _beginning_ at some of those extreme parts of it,
which reside in the muscles or organs of sense.

VOLITION is an exertion or change of the central parts of the sensorium, or
of the whole of it, _terminating_ in some of those extreme parts of it,
which reside in the muscles or organs of sense.

ASSOCIATION is an exertion or change of some extreme part of the sensorium
residing in the muscles or organs of sense, in consequence of some
antecedent or attendant fibrous contractions.

3. These four faculties of the animal sensorium may at the time of their
exertions be termed motions without impropriety of language; for we cannot
pass from a state of insensibility or inaction to a state of sensibility or
of exertion without some change of the sensorium, and every change includes
motion. We shall therefore sometimes term the above described faculties
_sensorial motions_ to distinguish them from _fibrous motions_; which
latter expression includes the motions of the muscles and organs of sense.

The active motions of the fibres, whether those of the muscles or organs of
sense, are probably simple contractions; the fibres being again elongated
by antagonist muscles, by circulating fluids, or sometimes by elastic
ligaments, as in the necks of quadrupeds. The sensorial motions, which
constitute the sensations of pleasure or pain, and which constitute
volition, and which cause the fibrous contractions in consequence of
irritation or of association, are not here supposed to be fluctuations or
refluctuations of the spirit of animation; nor are they supposed to be
vibrations or revibrations, nor condensations or equilibrations of it; but
to be changes or motions of it peculiar to life.

       *       *       *       *       *

SECT. VI.

OF THE FOUR CLASSES OF FIBROUS MOTIONS.

    I. _Origin of fibrous contractions._ II. _Distribution of them into
    four classes, irritative motions, sensitive motions, voluntary motions,
    and associate motions, defined._

I. All the fibrous contractions of animal bodies originate from the
sensorium, and resolve themselves into four classes, correspondent with the
four powers or motions of the sensorium above described, and from which
they have their causation.

1. These fibrous contractions were originally caused by the irritations
excited by objects, which are external to the moving organ. As the
pulsations of the heart are owing to the irritations excited by the
stimulus of the blood; and the ideas of perception are owing to the
irritations excited by external bodies.

2. But as painful or pleasurable sensations frequently accompanied those
irritations, by habit these fibrous contractions became causeable by the
sensations, and the irritations ceased to be necessary to their production.
As the secretion of tears in grief is caused by the sensation of pain; and
the ideas of imagination, as in dreams or delirium, are excited by the
pleasure or pain, with which they were formerly accompanied.

3. But as the efforts of the will frequently accompanied these painful or
pleasureable sensations, by habit the fibrous contractions became causable
by volition; and both the irritations and sensations ceased to be necessary
to their production. As the deliberate locomotions of the body, and the
ideas of recollection, as when we will to repeat the alphabet backwards.

4. But as many of these fibrous contractions frequently accompanied other
fibrous contractions, by habit they became causable by their associations
with them; and the irritations, sensations, and volition, ceased to be
necessary to their production. As the actions of the muscles of the lower
limbs in fencing are associated with those of the arms; and the ideas of
suggestion are associated with other ideas, which precede or accompany
them; as in repeating carelessly the alphabet in its usual order after
having began it.

II. We shall give the following names to these four classes of fibrous
motions, and subjoin their definitions.

1. Irritative motions. That exertion or change of the sensorium, which is
caused by the appulses of external bodies, either simply subsides, or is
succeeded by sensation, or it produces fibrous motions; it is termed
irritation, and irritative motions are those contractions of the muscular
fibres, or of the organs of sense, that are immediately consequent to this
exertion or change of the sensorium.

2. Sensitive motions. That exertion or change of the sensorium, which
constitutes pleasure or pain, either simply subsides, or is succeeded by
volition, or it produces fibrous motions; it is termed sensation, and the
sensitive motions are those contractions of the muscular fibres, or of the
organs of sense, that are immediately consequent to this exertion or change
of the sensorium.

3. Voluntary motions. That exertion or change of the sensorium, which
constitutes desire or aversion, either simply subsides, or is succeeded by
fibrous motions; it is then termed volition, and voluntary motions are
those contractions of the muscular fibres, or of the organs of sense, that
are immediately consequent to this exertion or change of the sensorium.

4. Associate motions. That exertion or change of the sensorium, which
accompanies fibrous motions, either simply subsides, or is succeeded by
sensation or volition, or it produces other fibrous motions; it is then
termed association, and the associate motions are those contractions of the
muscular fibres, or of the organs of sense, that are immediately consequent
to this exertion or change of the sensorium.

       *       *       *       *       *

SECT. VII.

OF IRRITATIVE MOTIONS.

    I. 1. _Some muscular motions are excited by perpetual irritations._ 2.
    _Others more frequently by sensations._ 3. _Others by volition. Case of
    involuntary stretchings in paralytic limbs._ 4. _Some sensual motions
    are excited by perpetual irritations._ 5. _Others more frequently by
    sensation or volition._

    II. 1. _Muscular motions excited by perpetual irritations occasionally
    become obedient sensation and to volition._ 2. _And the sensual
    motions._

    III. 1. _Other muscular motions are associated with the irritative
    ones._ 2. _And other ideas with irritative ones. Of letters, language,
    hieroglyphics. Irritative ideas exist without our attention to them._

I. 1. Many of our muscular motions are excited by perpetual irritations, as
those of the heart and arterial system by the circumfluent blood. Many
other of them are excited by intermitted irritations, as those of the
stomach and bowels by the aliment we swallow; of the bile-ducts by the
bile; of the kidneys, pancreas, and many other glands, by the peculiar
fluids they separate from the blood; and those of the lacteal and other
absorbent vessels by the chyle, lymph, and moisture of the atmosphere.
These motions are accelerated or retarded, as their correspondent
irritations are increased or diminished, without our attention or
consciousness, in the same manner as the various secretions of fruit, gum,
resin, wax, and, honey, are produced in the vegetable world, and as the
juices of the earth and the moisture of the atmosphere are absorbed by
their roots and foliage.

2. Other muscular motions, that are most frequently connected with our
sensations, as those of the sphincters of the bladder and anus, and the
musculi erectores penis, were originally excited into motion by irritation,
for young children make water, and have other evacuations without attention
to these circumstances; "et primis etiam ab incunabulis tenduntur sæpius
puerorum penes, amore nondum expergefacto." So the nipples of young women
are liable to become turgid by irritation, long before they are in a
situation to be excited by the pleasure of giving milk to the lips of a
child.

3. The contractions of the larger muscles of our bodies, that are most
frequently connected with volition, were originally excited into action by
internal irritations: as appears from the stretching or yawning of all
animals after long sleep. In the beginning of some fevers this irritation
of the muscles produces perpetual stretching and yawning; in other periods
of fever an universal restlessness arises from the same cause, the patient
changing the attitude of his body every minute. The repeated struggles of
the foetus in the uterus must be owing to this internal irritation: for the
foetus can have no other inducement to move its limbs but the tædium or
irksomeness of a continued posture.

The following case evinces, that the motions of stretching the limbs after
a continued attitude are not always owing to the power of the will. Mr.
Dean, a mason, of Austry in Leicestershire, had the spine of the third
vertebra of the back enlarged; in some weeks his lower extremities became
feeble, and at length quite paralytic: neither the pain of blisters, the
heat of fomentations, nor the utmost efforts of the will could produce the
least motion in these limbs; yet twice or thrice a day for many months his
feet, legs, and thighs, were affected for many minutes with forceable
stretchings, attended with the sensation of fatigue; and he at length
recovered the use of his limbs, though the spine continued protuberant. The
same circumstance is frequently seen in a less degree in the common
hemiplagia; and when this happens, I have believed repeated and strong
shocks of electricity to have been of great advantage.

4. In like manner the various organs of sense are originally excited into
motion by various external stimuli adapted to this purpose, which motions
are termed perceptions or ideas; and many of these motions during our
waking hours are excited by perpetual irritation, as those of the organs of
hearing and of touch. The former by the constant low indistinct noises that
murmur around us, and the latter by the weight of our bodies on the parts
which support them; and by the unceasing variations of the heat, moisture,
and pressure of the atmosphere; and these sensual motions, precisely as the
muscular ones above mentioned, obey their correspondent irritations without
our attention or consciousness.

5. Other classes of our ideas are more frequently excited by our sensations
of pleasure or pain, and others by volition: but that these have all been
originally excited by stimuli from external objects, and only vary in their
combinations or reparations, has been fully evinced by Mr. Locke: and are
by him termed the ideas of perception in contradistinction to those, which
he calls the ideas of reflection.

II. 1. These muscular motions, that are excited by perpetual irritation,
are nevertheless occasionally excitable by the sensations of pleasure or
pain, or by volition; as appears by the palpitation of the heart from fear,
the increased secretion of saliva at the sight of agreeable food, and the
glow on the skin of those who are ashamed. There is an instance told in the
Philosophical Transactions of a man, who could for a time stop the motion
of his heart when he pleased; and Mr. D. has often told me, be could so far
increase the peristaltic motion of his bowels by voluntary efforts, as to
produce an evacuation by stool at any time in half an hour.

2. In like manner the sensual motions, or ideas, that are excited by
perpetual irritation, are nevertheless occasionally excited by sensation or
volition; as in the night, when we listen under the influence of fear, or
from voluntary attention, the motions excited in the organ of hearing by
the whispering of the air in our room, the pulsation of our own arteries,
or the faint beating of a distant watch, become objects of perception.

III. 1. Innumerable trains or tribes of other motions are associated with
these muscular motions which are excited by irritation; as by the stimulus
of the blood in the right chamber of the heart, the lungs are induced to
expand themselves; and the pectoral and intercostal muscles, and the
diaphragm, act at the same time by their associations with them. And when
the pharinx is irritated by agreeable food, the muscles of deglutition are
brought into action by association. Thus when a greater light falls on the
eye, the iris is brought into action without our attention; and the ciliary
process, when the focus is formed before or behind the retina, by their
associations with the increased irritative motions of the organ of vision.
Many common actions of life are produced in a similar manner. If a fly
settle on my forehead, whilst I am intent on my present occupation, I
dislodge it with my finger, without exciting my attention or breaking the
train of my ideas.

2. In like manner the irritative ideas suggest to us many other trains or
tribes of ideas that are associated with them. On this kind of connection,
language, letters, hieroglyphics, and every kind of symbol, depend. The
symbols themselves produce irritative ideas, or sensual motions, which we
do not attend to; and other ideas, that are succeeded by sensation, are
excited by their association with them. And as these irritative ideas make
up a part of the chain of our waking thoughts, introducing other ideas that
engage our attention, though themselves are unattended to, we find it very
difficult to investigate by what steps many of our hourly trains of ideas
gain their admittance.

It may appear paradoxical, that ideas can exist, and not be attended to;
but all our perceptions are ideas excited by irritation, and succeeded by
sensation. Now when these ideas excited by irritation give us neither
pleasure nor pain, we cease to attend to them. Thus whilst I am walking
through that grove before my window, I do not run against the trees or the
benches, though my thoughts are strenuously exerted on some other object.
This leads us to a distinct knowledge of irritative ideas, for the idea of
the tree or bench, which I avoid, exists on my retina, and induces by
association the action of certain locomotive muscles; though neither itself
nor the actions of those muscles engage my attention.

Thus whilst we are conversing on this subject, the tone, note, and
articulation of every individual word forms its correspondent irritative
idea on the organ of hearing; but we only attend to the associated ideas,
that are attached by habit to these irritative ones, and are succeeded by
sensation; thus when we read the words "PRINTING-PRESS" we do not attend to
the shape, size, or existence of the letters which compose these words,
though each of them excites a correspondent irritative motion of our organ
of vision, but they introduce by association our idea of the most useful of
modern inventions; the capacious reservoir of human knowledge, whose
branching streams diffuse sciences, arts, and morality, through all nations
and all ages.

       *       *       *       *       *

SECT. VIII.

OF SENSITIVE MOTIONS.

    I. 1. _Sensitive muscular motions were originally excited into action
    by irritation._ 2. _And sensitive sensual motions, ideas of
    imagination, dreams._ II. 1. _Sensitive muscular motions are
    occasionally obedient to volition._ 2. _And sensitive sensual motions._
    III. 1. _Other muscular motions are associated with the sensitive
    ones._ 2. _And other sensual motions._

I. 1. Many of the motions of our muscles, that are excited into action by
irritation, are at the same time accompanied with painful or pleasurable
sensations; and at length become by habit causable by the sensations. Thus
the motions of the sphincters of the bladder and anus were originally
excited into action by irritation; for young children give no attention to
these evacuations; but as soon as they become sensible of the inconvenience
of obeying these irritations, they suffer the water or excrement to
accumulate, till it disagreeably affects them; and the action of those
sphincters is then in consequence of this disagreeable sensation. So the
secretion of saliva, which in young children is copiously produced by
irritation, and drops from their mouths, is frequently attended with the
agreeable sensation produced by the mastication of tasteful food;, till at
length the sight of such food to a hungry person excites into action these
salival glands; as is seen in the slavering of hungry dogs.

The motions of those muscles, which are affected by lascivious ideas, and
those which are exerted in smiling, weeping, starting from fear, and
winking at the approach of danger to the eye, and at times the actions of
every large muscle of the body become causable by our sensations. And all
these motions are performed with strength and velocity in proportion to the
energy of the sensation that excites them, and the quantity of sensorial
power.

2. Many of the motions of our organs of sense, or ideas, that were
originally excited into action by irritation, become in like manner more
frequently causable by our sensations of pleasure or pain. These motions
are then termed the ideas of imagination, and make up all the scenery and
transactions of our dreams. Thus when any painful or pleasurable sensations
possess us, as of love, anger, fear; whether in our sleep or waking hours,
the ideas, that have been formerly excited by the objects of these
sensations, now vividly recur before us by their connection with these
sensations themselves. So the fair smiling virgin, that excited your love
by her presence, whenever that sensation recurs, rises before you in
imagination; and that with all the pleasing circumstances, that had before
engaged your attention. And in sleep, when you dream under the influence of
fear, all the robbers, fires, and precipices, that you formerly have seen
or heard of, arise before you with terrible vivacity. All these sensual
motions, like the muscular ones above mentioned, are performed with
strength and velocity in proportion to the energy of the sensation of
pleasure or pain, which excites them, and the quantity of sensorial power.

II. 1. Many of these muscular motions above described, that are most
frequently excited by our sensations, are nevertheless occasionally
causable by volition; for we can smile or frown spontaneously, can make
water before the quantity or acrimony of the urine produces a disagreeable
sensation, and can voluntarily masticate a nauseous drug, or swallow a
bitter draught, though our sensation would strongly dissuade us.

2. In like manner the sensual motions, or ideas, that are most frequently
excited by our sensations, are nevertheless occasionally causeable by
volition, as we can spontaneously call up our last night's dream before us,
tracing it industriously step by step through all its variety of scenery
and transaction; or can voluntarily examine or repeat the ideas, that have
been excited by out disgust or admiration.

III. 1. Innumerable trains or tribes of motions are associated with these
sensitive muscular motions above mentioned; as when a drop of water falling
into the wind-pipe disagreeably affects the air-vessels of the lungs, they
are excited into violent action; and with these sensitive motions are
associated the actions of the pectoral and intercostal muscles, and the
diaphragm; till by their united and repeated succussions the drop is
returned through the larinx. The same occurs when any thing disagreeably
affects the nostrils, or the stomach, or the uterus; variety of muscles are
excited by association into forcible action, not to be suppressed by the
utmost efforts of the will; as in sneezing, vomiting, and parturition.

2. In like manner with these sensitive sensual motions, or ideas of
imagination, are associated many other trains or tribes of ideas, which by
some writers of metaphysics have been classed under the terms of
resemblance, causation, and contiguity; and will be more fully treated of
hereafter.

       *       *       *       *       *

SECT. IX.

OF VOLUNTARY MOTIONS.

    I. 1. _Voluntary muscular motions are originally excited by
    irritations._ 2. _And voluntary ideas. Of reason._ II. 1. _Voluntary
    muscular motions are occasionally causable by sensations._ 2. _And
    voluntary ideas._ III. 1. _Voluntary muscular motions are occasionally
    obedient to irritations._ 2. _And voluntary ideas._ IV. 1. _Voluntary
    muscular motions are associated with other muscular motions._ 2. _And
    voluntary ideas._

When pleasure or pain affect the animal system, many of its motions both
muscular and sensual are brought into action; as was shewn in the preceding
section, and were called sensitive motions. The general tendency of these
motions is to arrest and to possess the pleasure, or to dislodge or avoid
the pain: but if this cannot immediately be accomplished, desire or
aversion are produced, and the motions in consequence of this new faculty
of the sensorium are called voluntary.

I. 1. Those muscles of the body that are attached to bones, have in general
their principal connections with volition, as I move my pen or raise my
body. These motions were originally excited by irritation, as was explained
in the section on that subject, afterwards the sensations of pleasure or
pain, that accompanied the motions thus excited, induced a repetition of
them; and at length many of them were voluntarily practised in succession
or in combination for the common purposes of life, as in learning to walk,
or to speak; and are performed with strength and velocity in proportion to
the energy of the volition, that excites them, and the quantity of
sensorial power.

2. Another great class of voluntary motions consists of the ideas of
recollection. We will to repeat a certain train of ideas, as of the
alphabet backwards; and if any ideas, that do not belong to this intended
train, intrude themselves by other connections, we will to reject them, and
voluntarily persist in the determined train. So at my approach to a house
which I have but once visited, and that at the distance of many months, I
will to recollect the names of the numerous family I expect to see there,
and I do recollect them.

On this voluntary recollection of ideas our faculty of reason depends, as
it enables us to acquire an idea of the dissimilitude of any two ideas.
Thus if you voluntarily produce the idea of a right-angled triangle, and
then of a square; and after having excited these ideas repeatedly, you
excite the idea of their difference, which is that of another right-angled
triangle inverted over the former; you are said to reason upon this
subject, or to compare your ideas.

These ideas of recollection, like the muscular motions above mentioned,
were originally excited by the irritation of external bodies, and were
termed ideas of perception: afterwards the pleasure or pain, that
accompanied these motions, induced a repetition of them in the absence of
the external body, by which they were first excited; and then they were
termed ideas of imagination. At length they become voluntarily practised in
succession or in combination for the common purposes of life; as when we
make ourselves masters of the history of mankind, or of the sciences they
have investigated; and are then called ideas of recollection; and are
performed with strength and velocity in proportion to the energy of the
volition that excites them, and the quantity of sensorial power.

II. 1. The muscular motions above described, that are most frequently
obedient to the will are nevertheless occasionally causable by painful or
pleasurable sensation, as in the starting from fear, and the contraction of
the calf of the leg in the cramp.

2. In like manner the sensual motions, or ideas, that are most frequently
connected with volition, are nevertheless occasionally causable by painful
or pleasurable sensation. As the histories of men, or the description of
places, which we have voluntarily taken pains to remember, sometimes occur
to us in our dreams.

III. 1. The muscular motions that are generally subservient to volition,
are also occasionally causable by irritation, as in stretching the limbs
after sleep, and yawning. In this manner a contraction of the arm is
produced by passing the electric fluid from the Leyden phial along its
muscles; and that even though the limb is paralytic. The sudden motion of
the arm produces a disagreeable sensation in the joint, but the muscles
seem to be brought into action simply by irritation.

2. The ideas, that are generally subservient to the will, are in like
manner occasionally excited by irritation; as when we view again an object,
we have before well studied, and often recollected.

IV. 1. Innumerable trains or tribes of motions are associated with these
voluntary muscular motions above mentioned; as when I will to extend my arm
to a distant object, some other muscles are brought into action, and
preserve the balance of my body. And when I wish to perform any steady
exertion, as in threading a needle, or chopping with an ax, the pectoral
muscles are at the same time brought into action to preserve the trunk of
the body motionless, and we cease to respire for a time.

2. In like manner the voluntary sensual motions, or ideas of recollection,
are associated with many other trains or tribes of ideas. As when I
voluntarily recollect a gothic window, that I saw some time ago, the whole
front of the cathedral occurs to me at the same time.

       *       *       *       *       *

SECT. X.

OF ASSOCIATE MOTIONS.

    I. 1. _Many muscular motions excited by irritations in trains or tribes
    become associated._ 2. _And many ideas._ II. 1. _Many sensitive
    muscular motions become associated._ 2. _And many sensitive ideas._
    III. 1. _Many voluntary muscular motions become associated._ 2. _And
    then become obedient to sensation or irritation._ 3. _And many
    voluntary ideas become associated._

All the fibrous motions, whether muscular or sensual, which are frequently
brought into action together, either in combined tribes, or in successive
trains, become so connected by habit, that when one of them is reproduced
the others have a tendency to succeed or accompany it.

I. 1. Many of our muscular motions were originally excited in successive
trains, as the contractions of the auricles and of the ventricles of the
heart; and others in combined tribes, as the various divisions of the
muscles which compose the calf of the leg, which were originally irritated
into synchronous action by the tædium or irksomeness of a continued
posture. By frequent repetitions these motions acquire associations, which
continue during our lives, and even after the destruction of the greatest
part of the sensorium; for the heart of a viper or frog will continue to
pulsate long after it is taken from the body; and when it has entirely
ceased to move, if any part of it is goaded with a pin, the whole heart
will again renew its pulsations. This kind of connection we shall term
irritative association, to distinguish it from sensitive and voluntary
associations.

2. In like manner many of our ideas are originally excited in tribes; as
all the objects of sight, after we become so well acquainted with the laws
of vision, as to distinguish figure and distance as well as colour; or in
trains, as while we pass along the objects that surround us. The tribes
thus received by irritation become associated by habit, and have been
termed complex ideas by the writers of metaphysics, as this book, or that
orange. The trains have received no particular name, but these are alike
associations of ideas, and frequently continue during our lives. So the
taste of a pine-apple, though we eat it blindfold, recalls the colour and
shape of it; and we can scarcely think on solidity without figure.

II. 1. By the various efforts of our sensations to acquire or avoid their
objects, many muscles are daily brought into successive or synchronous
actions; these become associated by habit, and are then excited together
with great facility, and in many instances gain indissoluble connections.
So the play of puppies and kittens is a representation of their mode of
fighting or of taking their prey; and the motions of the muscles necessary
for those purposes become associated by habit, and gain a great adroitness
of action by these early repetitions: so the motions of the abdominal
muscles, which were originally brought into concurrent action, with the
protrusive motion of the rectum or bladder by sensation, become so
conjoined with them by habit, that they not only easily obey these
sensations occasioned by the stimulus of the excrement and urine, but are
brought into violent and unrestrainable action in the strangury and
tenesmus. This kind of connection we shall term sensitive association.

2. So many of our ideas, that have been excited together or in succession
by our sensations, gain synchronous or successive associations, that are
sometimes indissoluble but with life. Hence the idea of an inhuman or
dishonourable action perpetually calls up before us the idea of the wretch
that was guilty of it. And hence those unconquerable antipathies are
formed, which some people have to the sight of peculiar kinds of food, of
which in their infancy they have eaten to excess or by constraint.

III. 1. In learning any mechanic art, as music, dancing, or the use of the
sword, we teach many of our muscles to act together or in succession by
repeated voluntary efforts; which by habit become formed into tribes or
trains of association, and serve all our purposes with great facility, and
in some instances acquire an indissoluble union. These motions are
gradually formed into a habit of acting together by a multitude of
repetitions, whilst they are yet separately causable by the will, as is
evident from the long time that is taken up by children in learning to walk
and to speak; and is experienced by every one, when he first attempts to
skate upon the ice or to swim: these we shall term voluntary associations.

2. All these muscular movements, when they are thus associated into tribes
or trains, become afterwards not only obedient to volition, but to the
sensations and irritations; and the same movement composes a part of many
different tribes or trains of motion. Thus a single muscle, when it acts in
consort with its neighbours on one side, assists to move the limb in one
direction; and in another, when, it acts with those in its neighbourhood on
the other side; and in other directions, when it acts separately or jointly
with those that lie immediately under or above it; and all these with equal
facility after their associations have been well established.

The facility, with which each muscle changes from one associated tribe to
another, and that either backwards or forwards, is well observable in the
muscles of the arm in moving the windlass of an air-pump; and the slowness
of those muscular movements, that have not been associated by habit, may be
experienced by any one, who shall attempt to saw the air quick
perpendicularly with one hand, and horizontally with the other at the same
time.

3. In learning every kind of science we voluntarily associate many tribes
and trains of ideas, which afterwards are ready for all the purposes either
of volition, sensation, or irritation; and in some instances acquire
indissoluble habits of acting together, so as to affect our reasoning, and
influence our actions. Hence the necessity of a good education.

These associate ideas are gradually formed into habits of acting together
by frequent repetition, while they are yet separately obedient to the will;
as is evident from the difficulty we experience in gaining so exact an idea
of the front of St. Paul's church, as to be able to delineate it with
accuracy, or in recollecting a poem of a few pages.

And these ideas, thus associated into tribes, not only make up the parts of
the trains of volition, sensation, and irritation; but the same idea
composes a part of many different tribes and trains of ideas. So the simple
idea of whiteness composes a part of the complex idea of snow, milk, ivory;
and the complex idea of the letter A composes a part of the several
associated trains of ideas that make up the variety of words, in which this
letter enters.

The numerous trains of these associated ideas are divided by Mr. Hume into
three classes, which he has termed contiguity, causation, and resemblance.
Nor should we wonder to find them thus connected together, since it is the
business of our lives to dispose them into those three classes; and we
become valuable to ourselves and our friends, as we succeed in it. Those
who have combined an extensive class of ideas by the contiguity of time or
place, are men learned in the history of mankind, and of the sciences they
have cultivated. Those who have connected a great class of ideas of
resemblances, possess the source of the ornaments of poetry and oratory,
and of all rational analogy. While those who have connected great classes
of ideas of causation, are furnished with the powers of producing effects.
These are the men of active wisdom, who lead armies to victory, and
kingdoms to prosperity; or discover and improve the sciences, which
meliorate and adorn the condition of humanity.

       *       *       *       *       *

SECT. XI.

ADDITIONAL OBSERVATIONS ON THE SENSORIAL POWERS.

    I. _Stimulation is of various kinds adapted to the organs of sense, to
    the muscles, to hollow membranes, and glands. Some objects irritate our
    senses by repeated impulses._ II. 1. _Sensation and volition frequently
    affect the whole sensorium._ 2. _Emotions, passions, appetites._ 3.
    _Origin of desire and aversion. Criterion of voluntary actions,
    difference of brutes and men._ 4. _Sensibility and voluntarity._ III.
    _Associations formed before nativity, irritative motions mistaken for
    officiated ones._

_Irritation._

I. The various organs of sense require various kinds of stimulation to
excite them into action; the particles of light penetrate the cornea and
humours of the eye, and then irritate the naked retina; rapid particles,
dissolved or diffused in water or saliva, and odorous ones, mixed or
combined with the air, irritate the extremities of the nerves of taste and
smell; which either penetrate, or are expanded on the membranes of the
tongue and nostrils; the auditory nerves are stimulated by the vibrations
of the atmosphere communicated by means of the tympanum and of the fluid,
whether of air or of water, behind it; and the nerves of touch by the
hardness of surrounding bodies, though the cuticle is interposed between
these bodies and the medulla of the nerve.

As the nerves of the senses have each their appropriated objects, which
stimulate them into activity; so the muscular fibres, which are the
terminations of other sets of nerves, have their peculiar objects, which
excite them into action; the longitudinal muscles are stimulated into
contraction by extension, whence the stretching or pandiculation after a
long continued posture, during which they have been kept in a state of
extension; and the hollow muscles are excited into action by distention, as
those of the rectum and bladder are induced to protrude their contents from
their sense of the distention rather than of the acrimony of those
contents.

There are other objects adapted to stimulate the nerves, which terminate in
variety of membranes, and those especially which form the terminations of
canals; thus the preparations of mercury particularly affect the salivary
glands, ipecacuanha the stomach, aloe the sphincter of the anus,
cantharides that of the bladder, and lastly every gland of the body appears
to be indued with a kind of taste, by which it selects or forms each its
peculiar fluid from the blood; and by which it is irritated into activity.

Many of these external properties of bodies, which stimulate our organs of
sense, do not seem to effect this by a single impulse, but by repeated
impulses; as the nerve of the ear is probably not excitable by a single
vibration of air, nor the optic nerve by a single particle of light; which
circumstance produces some analogy between those two senses, at the same
time the solidity of bodies is perceived by a single application of a solid
body to the nerves of touch, and that even through the cuticle; and we are
probably possessed of a peculiar sense to distinguish the nice degrees of
heat and cold.

The senses of touch and of hearing acquaint us with the mechanical impact
and vibration of bodies, those of smell and taste seem to acquaint us with
some of their chemical properties, while the sense of vision and of heat
acquaint us with the existence of their peculiar fluids.

_Sensation and Volition._

II. Many motions are produced by pleasure or pain, and that even in
contradiction to the power of volition, as in laughing, or in the
strangury; but as no name has been given to pleasure or pain, at the time
it is exerted so as to cause fibrous motions, we have used the term
sensation for this purpose; and mean it to bear the same analogy to
pleasure and pain, that the word volition does to desire and aversion.

1. It was mentioned in the fifth Section, that, what we have termed
sensation is a motion of the central parts, or of the whole sensorium,
_beginning_ at some of the extremities of it. This appears first, because
our pains and pleasures are always caused by our ideas or muscular motions,
which are the motions of the extremities of the sensorium. And, secondly,
because the sensation of pleasure or pain frequently continues some time
after the ideas or muscular motions which excited it have ceased: for we
often feel a glow of pleasure from an agreeable reverie, for many minutes
after the ideas, that were the subject of it, have escaped our memory; and
frequently experience a dejection of spirits without being able to assign
the cause of it but by much recollection.

When the sensorial faculty of desire or aversion is exerted so as to cause
fibrous motions, it is termed volition; which is said in Sect. V. to be a
motion of the central parts, or of the whole sensorium, _terminating_ in
some of the extremities of it. This appears, first, because our desires and
aversions always terminate in recollecting and comparing our ideas, or in
exerting our muscles; which are the motions of the extremities of the
sensorium. And, secondly, because desire or aversion begins, and frequently
continues for a time in the central parts of the sensorium, before it is
peculiarly exerted at the extremities of it; for we sometimes feel desire
or aversion without immediately knowing their objects, and in consequence
without immediately exerting any of our muscular or sensual motions to
attain them: as in the beginning of the passion of love, and perhaps of
hunger, or in the ennui of indolent people.

Though sensation and volition begin or terminate at the extremities or
central parts of the sensorium, yet the whole of it is frequently
influenced by the exertion of these faculties, as appears from their
effects on the external habit: for the whole skin is reddened by shame, and
an universal trembling is produced by fear: and every muscle of the body is
agitated in angry people by the desire of revenge.

There is another very curious circumstance, which shews that sensation and
volition are movements of the sensorium in contrary directions; that is,
that volition begins at the central parts of it, and proceeds to the
extremities; and that sensation begins at the extremities, and proceeds to
the central parts: I mean that these two sensorial faculties cannot be
strongly exerted at the same time; for when we exert our volition strongly,
we do not attend to pleasure or pain; and conversely, when we are strongly
affected with the sensation of pleasure or pain, we use no volition. As
will be further explained in Section XVIII. on sleep, and Section XXXIV. on
volition.

2. All our emotions and passions seem to arise out of the exertions of
these two faculties of the animal sensorium. Pride, hope, joy, are the
names of particular pleasures: shame, despair, sorrow, are the names of
peculiar pains: and love, ambition, avarice, of particular desires: hatred,
disgust, fear, anxiety, of particular aversions. Whilst the passion of
anger includes the pain from a recent injury, and the aversion to the
adversary that occasioned it. And compassion is the pain we experience at
the sight of misery, and the desire of relieving it.

There is another tribe of desires, which are commonly termed appetites, and
are the immediate consequences of the absence of some irritative motions.
Those, which arise from defect of internal irritations, have proper names
conferred upon them, as hunger, thirst, lust, and the desire of air, when
our respiration is impaired by noxious vapours; and of warmth, when we are
exposed to too great a degree of cold. But those, whose stimuli are
external to the body, are named from the objects, which are by nature
constituted to excite them; these desires originate from our past
experience of the pleasurable sensations they occasion, as the smell of an
hyacinth, or the taste of a pine-apple.

Whence it appears, that our pleasures and pains are at least as various and
as numerous as our irritations; and that our desires and aversions must be
as numerous as our pleasures and pains. And that as sensation is here used
as a general term for our numerous pleasures and pains, when they produce
the contractions of our fibres; so volition is the general name for our
desires and aversions, when they produce fibrous contractions. Thus when a
motion of the central parts, or of the whole sensorium, terminates in the
exertion of our muscles, it is generally called voluntary action; when it
terminates in the exertion of our ideas, it is termed recollection,
reasoning, determining.

3. As the sensations of pleasure and pain are originally introduced by the
irritations of external objects: so our desires and aversions are
originally introduced by those sensations; for when the objects of our
pleasures or pains are at a distance, and we cannot instantaneously possess
the one, or avoid the other, then desire or aversion is produced, and a
voluntary exertion of our ideas or muscles succeeds.

The pain of hunger excites you to look out for food, the tree, that shades
you, presents its odoriferous fruit before your eyes, you approach, pluck,
and eat.

The various movements of walking to the tree, gathering the fruit, and
masticating it, are associated motions introduced by their connection with
sensation; but if from the uncommon height of the tree, the fruit be
inaccessible, and you are prevented from quickly possessing the intended
pleasure, desire is produced. The consequence of this desire is, first, a
deliberation about the means to gain the object of pleasure in process of
time, as it cannot be procured immediately; and, secondly, the muscular
action necessary for this purpose.

You voluntarily call up all your ideas of causation, that are related to
the effect you desire, and voluntarily examine and compare them, and at
length determine whether to ascend the tree, or to gather stones from the
neighbouring brook, is easier to practise, or more promising of success;
and, finally, you gather the stones, and repeatedly fling them to dislodge
the fruit.

Hence then we gain a criterion to distinguish voluntary acts or thoughts
from those caused by sensation. As the former are always employed about the
_means_ to acquire pleasurable objects, or the _means_ to avoid painful
ones; while the latter are employed in the possession of those, which are
already in our power.

Hence the activity of this power of volition produces the great difference
between the human and the brute creation. The ideas and the actions of
brutes are almost perpetually employed about their present pleasures, or
their present pains; and, except in the few instances which are mentioned
in Section XVI, on instinct, they seldom busy themselves about the means of
procuring future bliss, or of avoiding future misery; so that the acquiring
of languages, the making of tools, and labouring for money, which are all
only the means to procure pleasures; and the praying to the Deity, as
another means to procure happiness, are characteristic of human nature.

4. As there are many diseases produced by the quantity of the sensation of
pain or pleasure being too great or too little; so are there diseases
produced by the susceptibility of the constitution to motions causable by
these sensations being too dull or too vivid. This susceptibility of the
system to sensitive motions is termed sensibility, to distinguish it from
sensation, which is the actual existence or exertion of pain or pleasure.

Other classes of diseases are owing to the excessive promptitude, or
sluggishness of the constitution to voluntary exertions, as well as to the
quantity of desire or of aversion. This susceptibility of the system to
voluntary motions is termed voluntarity, to distinguish it from volition,
which is the exertion of desire or aversion; these diseases will be treated
of at length in the progress of the work.

_Association._

III. 1. It is not easy to assign a cause, why those animal movements, that
have once occurred in succession, or in combination, should afterwards have
a tendency to succeed or accompany each other. It is a property of
animation, and distinguishes this order of being from the other productions
of nature.

When a child first wrote the word man, it was distinguished in his mind
into three letters, and those letters into many parts of letters; but by
repeated use the word man becomes to his hand in writing it, as to his
organs of speech in pronouncing it, but one movement without any
deliberation, or sensation, or irritation, interposed between the parts of
it. And as many separate motions of our muscles thus become united, and
form, as it were, one motion; so each separate motion before such union may
be conceived to consist of many parts or spaces moved through; and perhaps
even the individual fibres of our muscles have thus gradually been brought
to act in concert, which habits began to be acquired as early as the very
formation of the moving organs, long before the nativity of the animal; as
explained in the Section XVI. 2. on instinct.

2. There are many motions of the body, belonging to the irritative class,
which might by a hasty observer be mistaken for associated ones; as the
peristaltic motion of the stomach and intestines, and the contractions of
the heart and arteries, might be supposed to be associated with the
irritative motions of their nerves of sense, rather than to be excited by
the irritation of their muscular fibres by the distention, acrimony, or
momentum of the blood. So the distention or elongation of muscles by
objects external to them irritates them into contraction, though the
cuticle or other parts may intervene between the stimulating body and the
contracting muscle. Thus a horse voids his excrement when its weight or
bulk irritates the rectum or sphincter ani. These muscles act from the
irritation of distention, when he excludes his excrement, but the muscles
of the abdomen and diaphragm are brought into motion by association with
those of the sphincter and rectum.

       *       *       *       *       *

SECT. XII.

OF STIMULUS, SENSORIAL EXERTION, AND FIBROUS CONTRACTION.

    I. Of fibrous contraction. 1. _Two particles of a fibre cannot approach
    without the intervention of something, as in magnetism, electricity,
    elasticity. Spirit of life is not electric ether. Galvani's
    experiments._ 2. _Contraction of a fibre._ 3. _Relaxation succeeds._ 4.
    _Successive contractions, with intervals. Quick pulse from debility,
    from paucity of blood. Weak contractions performed in less time, and
    with shorter intervals._ 5. _Last situation of the fibres continues
    after contraction._ 6. _Contraction greater than usual induces pleasure
    or pain._ 7. _Mobility of the fibres uniform. Quantity of sensorial
    power fluctuates. Constitutes excitability._ II. Of sensorial exertion.
    1. _Animal motion includes stimulus, sensorial power, and contractile
    fibres. The sensorial faculties act separately or conjointly. Stimulus
    of four kinds. Strength and weakness defined. Sensorial power
    perpetually exhausted and renewed. Weakness from defect of stimulus.
    From defect of sensorial power, the direct and indirect debility of Dr.
    Brown. Why we become warm in Buxton bath after a time, and see well
    after a time in a darkish room. Fibres may act violently, or with their
    whole force, and yet feebly. Great exertion in inflammation explained.
    Great muscular force of some insane people._ 2. _Occasional
    accumulation of sensorial power in muscles subject to constant
    stimulus. In animals sleeping in winter. In eggs, seeds, schirrous
    tumours, tendons, bones._ 3. _Great exertion introduces pleasure or
    pain. Inflammation. Libration of the system between torpor and
    activity. Fever-fits._ 4. _Desire and aversion introduced. Excess of
    volition cures fevers._ III. Of repeated stimulus. 1. _A stimulus
    repeated too frequently looses effect. As opium, wine, grief. Hence old
    age. Opium and aloes in small doses._ 2. _A stimulus not repeated too
    frequently does not lose effect. Perpetual movement of the vital
    organs._ 3. _A stimulus repeated at uniform times produces greater
    effect. Irritation combined with association._ 4. _A stimulus repeated
    frequently and uniformly may be withdrawn, and the action of the organ
    will continue. Hence the bark cures agues, and strengthens weak
    constitutions._ 5. _Defect of stimulus repeated at certain intervals
    causes fever-fits._ 6. _Stimulus long applied ceases to act a second
    time._ 7. _If a stimulus excites sensation in an organ not usually
    excited into sensation, inflammation is produced._ IV. Of stimulus
    greater than natural. 1. _A stimulus greater than natural diminishes
    the quantity of sensorial power in general._ 2. _In particular organs._
    3. _Induces the organ into spasmodic actions._ 4. _Induces the
    antagonist fibres into action._ 5. _Induces the organ into convulsive
    or fixed spasms._ 6. _Produces paralysis of the organ._ V. Of stimulus
    less than natural. 1. _Stimulus less than natural occasions
    accumulation of sensorial power in general._ 2. _In particular organs,
    flushing of the face in a frosty morning. In fibres subject to
    perpetual stimulus only. Quantity of sensorial power inversely as the
    stimulus._ 3. _Induces pain. As of cold, hunger, head-ach._ 4. _Induces
    more feeble and frequent contraction. As in low fevers. Which are
    frequently owing to deficiency of sensorial power rather than to
    deficiency of stimulus._ 5. _Inverts successive trains of motion.
    Inverts ideas._ 6. _Induces paralysis and death._ VI. Cure of increased
    exertion. 1. _Natural cure of exhaustion of sensorial power._ 2.
    _Decrease the irritations. Venesection. Cold. Abstinence._ 3. _Prevent
    the previous cold fit. Opium. Bark. Warmth. Anger. Surprise._ 4.
    _Excite some other part of the system. Opium and warm bath relieve
    pains both from defect and from excess of stimulus._ 5. _First increase
    the stimulus above, and then decrease it beneath the natural quantity._
    VII. Cure of decreased exertion. 1. _Natural cure by accumulation of
    sensorial power. Ague-fits. Syncope._ 2. _Increase the stimulation, by
    wine, opium, given so as not to intoxicate. Cheerful ideas._ 3. _Change
    the kinds of stimulus._ 4. _Stimulate the associated organs. Blisters
    of use in heart-burn, and cold extremities._ 5. _Decrease the
    stimulation for a time, cold bath._ 6. _Decrease the stimulation below
    natural, and then increase it above natural. Bark after emetics. Opium
    after venesection. Practice of Sydenham in chlorosis._ 7. _Prevent
    unnecessary expenditure of sensorial power. Decumbent posture, silence,
    darkness. Pulse quickened by rising out of bed._ 8. _To the greatest
    degree of quiescence apply the least stimulus. Otherwise paralysis or
    inflammation of the organ ensues. Gin, wine, blisters, destroy by too
    great stimulation in fevers with debility. Intoxication in the
    slightest degree succeeded by debility. Golden rule for determining the
    best degree of stimulus in low fevers. Another golden rule for
    determining the quantity of spirit which those, who are debilitated by
    drinking it, may safely omit._

I. _Of fibrous contraction._

1. If two particles of iron lie near each other without motion, and
afterwards approach each other; it is reasonable to conclude that something
besides the iron particles is the cause of their approximation; this
invisible something is termed magnetism. In the same manner, if the
particles, which compose an animal muscle, do not touch each other in the
relaxed state of the muscle, and are brought into contact during the
contraction of the muscle, it is reasonable to conclude, that some other
agent is the cause of this new approximation. For nothing can act, where it
does not exist; for to act includes to exist; and therefore the particles
of the muscular fibre (which in its state of relaxation are supposed not to
touch) cannot affect each other without the influence of some intermediate
agent; this agent is here termed the spirit of animation, or sensorial
power, but may with equal propriety be termed the power, which causes
contraction; or may be called by any other name, which the reader may
choose to affix to it.

The contraction of a muscular fibre may be compared to the following
electric experiment, which is here mentioned not as a philosophical
analogy, but as an illustration or simile to facilitate the conception of a
difficult subject. Let twenty very small Leyden phials properly coated be
hung in a row by fine silk threads at a small distance from each other; let
the internal charge of one phial be positive, and of the other negative
alternately, if a communication be made from the internal surface of the
first to the external surface of the last in the row, they will all of them
instantly approach each other, and thus shorten a line that might connect
them like a muscular fibre. See Botanic Garden, p. 1. Canto I. 1. 202, note
on Gymnotus.

The attractions of electricity or of magnetism do not apply philosophically
to the illustration of the contraction of animal fibres, since the force of
those attractions increases in some proportion inversely as the distance,
but in muscular motion there appears no difference in velocity or strength
during the beginning or end of the contraction, but what may be clearly
ascribed to the varying mechanic advantage in the approximation of one bone
to another. Nor can muscular motion be assimilated with greater
plausibility to the attraction of cohesion or elasticity; for in bending a
steel spring, as a small sword, a less force is required to bend it the
first inch than the second; and the second than the third; the particles of
steel on the convex side of the bent spring endeavouring to restore
themselves more powerfully the further they are drawn from each other. See
Botanic Garden, P. I. addit. Note XVIII.

I am aware that this may be explained another way, by supposing the
elasticity of the spring to depend more on the compression of the particles
on the concave side than on the extension of them on the convex side; and
by supposing the elasticity of the elastic gum to depend more on the
resistance to the lateral compression of its particles than to the
longitudinal extension of them. Nevertheless in muscular contraction, as
above observed, there appears no difference in the velocity or force of it
at its commencement or at its termination; from whence we must conclude
that animal contraction is governed by laws of its own, and not by those of
mechanics, chemistry, magnetism, or electricity.

On these accounts I do not think the experiments conclusive, which were
lately published by Galvani, Volta, and others, to shew a similitude
between the spirit of animation, which contracts the muscular fibres, and
the electric fluid. Since the electric fluid may act only as a more potent
stimulus exciting the muscular fibres into action, and not by supplying
them with a new quantity of the spirit of life. Thus in a recent hemiplegia
I have frequently observed, when the patient yawned and stretched himself,
that the paralytic limbs moved also, though they were totally disobedient
to the will. And when he was electrified by passing shocks from the
affected hand to the affected foot, a motion of the paralytic limbs was
also produced. Now as in the act of yawning the muscles of the paralytic
limbs were excited into action by the stimulus of the irksomeness of a
continued posture, and not by any additional quantity of the spirit of
life; so we may conclude, that the passage of the electric fluid, which
produced a similar effect, acted only as a stimulus, and not by supplying
any addition of sensorial power.

If nevertheless this theory should ever become established, a stimulus must
be called an eductor of vital ether; which stimulus may consist of
sensation or volition, as in the electric eel, as well as in the appulses
of external bodies; and by drawing off the charges of vital fluid may
occasion the contraction or motions of the muscular fibres, and organs of
sense.

2. The immediate effect of the action of the spirit of animation or
sensorial power on the fibrous parts of the body, whether it acts in the
mode of irritation, sensation, volition, or association, is a contraction
of the animal fibre, according to the second law of animal causation. Sect.
IV. Thus the stimulus of the blood induces the contraction of the heart;
the agreeable taste of a strawberry produces the contraction of the muscles
of deglutition; the effort of the will contracts the muscles, which move
the limbs in walking; and by association other muscles of the trunk are
brought into contraction to preserve the balance of the body. The fibrous
extremities of the organs of sense have been shewn, by the ocular spectra
in Sect. III. to suffer similar contraction by each of the above modes of
excitation; and by their configurations to constitute our ideas.

3. After animal fibres have for some time been excited into contraction, a
relaxation succeeds, even though the exciting cause continues to act. In
respect to the irritative motions this is exemplified in the peristaltic
contractions of the bowels; which cease and are renewed alternately, though
the stimulus of the aliment continues to be uniformly applied; in the
sensitive motions, as in strangury, tenesmus, and parturition, the
alternate contractions and relaxations of the muscles exist, though the
stimulus is perpetual. In our voluntary exertions it is experienced, as no
one can hang long by the hands, however vehemently he wills so to do; and
in the associate motions the constant change of our attitudes evinces the
necessity of relaxation to those muscles, which have been long in action.

This relaxation of a muscle after its contraction, even though the stimulus
continues to be applied, appears to arise from the expenditure or
diminution of the spirit of animation previously resident in the muscle,
according to the second law of animal causation in Sect. IV. In those
constitutions, which are termed weak, the spirit of animation becomes
sooner exhausted, and tremulous motions are produced, as in the hands of
infirm people, when they lift a cup to their mouths. This quicker
exhaustion of the spirit of animation is probably owing to a less quantity
of it residing in the acting fibres, which therefore more frequently
require a supply from the nerves, which belong to them.

4. If the sensorial power continues to act, whether it acts in the mode of
irritation, sensation, volition, or association, a new contraction of the
animal fibre succeeds after a certain interval; which interval is of
shorter continuance in weak people than in strong ones. This is exemplified
in the shaking of the hands of weak people, when they attempt to write. In
a manuscript epistle of one of my correspondents, which is written in a
small hand, I observed from four to six zigzags in the perpendicular stroke
of every letter, which shews that both the contractions of the fingers, and
intervals between them, must have been performed in very short periods of
time.

The times of contraction of the muscles of enfeebled people being less, and
the intervals between those contractions being less also, accounts for the
quick pulse in fevers with debility, and in dying animals. The shortness of
the intervals between one contraction and another in weak constitutions, is
probably owing to the general deficiency of the quantity of the spirit of
animation, and that therefore there is a less quantity of it to be received
at each interval of the activity of the fibres. Hence in repeated motions,
as of the fingers in performing on the harpsichord, it would at first sight
appear, that swiftness and strength were incompatible; nevertheless the
single contraction of a muscle is performed with greater velocity as well
as with greater force by vigorous constitutions, as in throwing a javelin.

There is however another circumstance, which may often contribute to cause
the quickness of the pulse in nervous fevers, as in animals bleeding to
death in the slaughter-house; which is the deficient quantity of blood;
whence the heart is but half distended, and in consequence sooner
contracts. See Sect. XXXII. 2. 1.

For we must not confound frequency of repetition with quickness of motion,
or the number of pulsations with the velocity, with which the fibres, which
constitute the coats of the arteries, contract themselves. For where the
frequency of the pulsations is but seventy-five in a minute, as in health;
the contracting fibres, which constitute the sides of the arteries, may
move through a greater space in a given time, than where the frequency of
pulsation is one hundred and fifty in a minute, as in some fevers with
great debility. For if in those fevers the arteries do not expand
themselves in their diastole to more than half the usual diameter of their
diastole in health, the fibres which constitute their coats, will move
through a less space in a minute than in health, though they make two
pulsations for one.

Suppose the diameter of the artery during its systole to be one line, and
that the diameter of the same artery during its diastole is in health is
four lines, and in a fever with, great debility only two lines. It follows,
that the arterial fibres contract in health from a circle of twelve lines
in circumference to a circle of three lines in circumference, that is they
move through a space of nine lines in length. While the arterial fibres in
the fever with debility would twice contract from a circle of six lines to
a circle of three lines; that is while they move through a space equal to
six lines. Hence though the frequency of pulsation in fever be greater as
two to one, yet the velocity of contraction in health is greater as nine to
six, or as three to two.

On the contrary in inflammatory diseases with strength, as in the pleurisy,
the velocity of the contracting sides of the arteries is much greater than
in health, for if we suppose the number of pulsations in a pleurisy to be
half as much more than in health, that is as one hundred and twenty to
eighty, (which is about what generally happens in inflammatory diseases)
and if the diameter of the artery in diastole be one third greater than in
health, which I believe is near the truth, the result will be, that the
velocity of the contractile sides of the arteries will be in a pleurisy as
two and a half to one, compared to the velocity of their contraction in a
state of health, for if the circumference of the systole of the artery be
three lines, and the diastole in health be twelve lines in circumference,
and in a pleurisy eighteen lines; and secondly, if the artery pulsates
thrice in the diseased state for twice in the healthy one, it follows, that
the velocity of contraction in the diseased state to that in the healthy
state will be forty-five to eighteen, or as two and a half to one.

From hence it would appear, that if we had a criterion to determine the
velocity of the arterial contractions, it would at the same time give us
their strength, and thus be of more service in distinguishing diseases,
than the knowledge of their frequency. As such a criterion cannot be had,
the frequency of pulsation, the age of the patient being allowed for, will
in some measure assist us to distinguish arterial strength from arterial
debility, since in inflammatory diseases with strength the frequency seldom
exceeds one hundred and eighteen or one hundred and twenty pulsations in a
minute; unless under peculiar circumstance, as the great additional stimuli
of wine or of external heat.

5. After a muscle or organ of sense has been excited into contraction, and
the sensorial power ceases to act, the last situation or configuration of
it continues; unless it be disturbed by the action of some antagonist
fibres, or other extraneous power. Thus in weak or languid people, wherever
they throw their limbs on their bed or sofa, there they lie, till another
exertion changes their attitude; hence one kind of ocular spectra seems to
be produced after looking at bright objects; thus when a fire-stick is
whirled round in the night, there appears in the eye a complete circle of
fire; the action o