THE LAWS OF NERVOUS ACTIVITY.
161. The foregoing remarks have had the object of showing how little
substantial aid Psychology can at present derive from what is known
of the elementary structure of the nervous system, indispensable
as an accurate knowledge of that structure must be to a complete
analysis of its functions. This caution has been specially addressed
to those medical and psychological students whose researches leave
them insufficient leisure to pursue microscopical investigations
for themselves, and who are therefore forced to rely on second-hand
knowledge, which is usually defective in the many qualifying
considerations which keep scepticism vigilant. Relying on positive
statements, and delusive diagrams which only display what the observer
_imagines_, not what he actually _sees_, they construct on such data
theories of disease, or of mental processes; or else they translate
observed facts into the terms of this imaginary anatomy, and offer the
translation as a new contribution to Science.
162. But little aid as can at present be derived from the teaching of
the microscope, some aid Psychology may even now derive from it. The
teaching will often serve, for instance, to correct the precipitate
conclusions of subjective analysis, which present artificial
distinctions as real distinctions, separating what Nature has united.
It will show certain organic connections not previously suspected;
and since whatever is organically connected cannot functionally be
separated, such sharply marked analytical distinctions as those of
periphery and centre, or of sensation and motion, must be only regarded
as artificial aids. The demonstration of the indissoluble union of
the tissues is a demonstration of their functional co-operation. So
also the anatomical demonstration of the similarity and continuity of
all parts of the central system sets aside the analytical separation
of one centre from another, except as a convenient artifice; proving
that cerebral substance is one with spinal substance, having the same
properties, the same laws of action.
For the present, Psychology must seek objective aid from Physiology
and Pathology rather than from elementary Anatomy. In the paragraphs
which are to follow I shall endeavor to select the chief laws of
nervous activity which the researches of physiologists and pathologists
disclose. By these laws we may direct and control psychological
research.
THE ENERGY OF NEURILITY.
163. Vitality is characterized by incessant molecular movement, both
of composition and decomposition, in the building up of structure and
the liberation of energy. The life of every organism is a complex of
changes, each of which directly or indirectly affects the statical
and dynamical relations, each being the resultant of many co-operant
forces. In the nourishment of every organite there is an accumulation
of molecular _tension_, that is to say, stored-up energy in a latent
state, ready to be expended in the activity of that organite; and this
expenditure may take place in a steady flow, or in a sudden gush.
The molecular movements under one aspect may be called convergent,
or _formative_: they build the structure, and tend to the state of
equilibrium which we call the statical condition of the organite,
i. e. the condition in which it is not active, but ready to act.
Perfect equilibrium is of course never attained, owing to the
incessant molecular change: indeed Life is inconsistent with complete
repose. Under another aspect the molecular movements may be called
_discharging_: they constitute the dynamic condition of the organite,
in which its functional activity appears. The energy is now diverted,
liberated, and the surplus, over and above that which is absorbed in
formation, instead of slowly dribbling off, gushes forth in a directed
stream. The slow formation of a secretion in a gland-cell, and the
discharge of that secretion, will illustrate this; or (if muscular
_tone_ be admitted) the incipient contraction of the chronic state, and
the complete contraction of the dynamic state, may also be cited.
164. The discharge which follows excitation may thus be viewed as
a _directed quantity_ of molecular movement. Because it is always
strictly relative to the energy of tension, and is inevitable when
that tension attains a certain surplus over what is required in
construction, there is a limit, 1°, to the growth and evolution of
every organite, and every organism (comp. Problem I. § 118), and, 2°,
to its dynamical effect. When there is no surplus, the organite is
incapable of discharge: it is then exhausted, i. e. will not respond to
stimulus.
165. The speciality of nerve-tissue is its pre-eminence in directive
energy. Like all other tissues, it grows, develops, and dies; but
above all others it has what we call _excitability_, or readiness in
discharging its energy in a directed stream. By its topographical
distribution it plays the functional part of exciting the activity of
other tissues: it transmits molecular disturbance from periphery to
centre, from centre to centre, and from centre to muscles, vessels, and
glands. When a muscle is excited it moves, and when a gland is excited
it secretes; but these actions end, so to speak, with themselves; the
muscle does not directly move any other muscle;[184] the gland does not
directly excite any other gland. The nerve, on the contrary, has always
a wide-spreading effect; it excites a centre which is continuous with
other centres; and in exciting one muscle, usually excites a group.
Hence the nervous system is that _which binds the different organs
into a dynamic unity_. And Comparative Anatomy teaches that there is a
parallelism between the development of this system and the efficient
complexity of the organism. As the tissues become more and more
specialized, and the organs more and more individualized, they would
become more and more unsuited to the general service of the organism,
were it not that a corresponding development of the nervous system
brought a unifying mechanism.
The great instability of neurine, in other words, its high degree of
tension, renders it especially apt to disturb the tension of other
tissues. It is very variable; and this variability will have to be
taken into account in explaining the restriction of discharges to
particular centres. A good example of exaggerated tension is furnished
by strychnine poisoning. The centres are then so readily excitable
that a touch, or a puff of cold air on the skin, will determine
convulsions. And it is worthy of remark that for some hours after this
convulsive discharge the centres return to something like their normal
state; and the animal may then be stroked, pinched, or blown upon
without abnormal reactions. But during this interval the centres are
slowly accumulating excess of tension from the poisoned blood; and at
the close, convulsions will again follow the slightest stimulus. This
alternation of exhaustion and recrudescence is noticed by Schröder
van der Kolk in the periodicity of the phenomena exhibited in spinal
disease.[185]
THE PROPAGATION OF EXCITATION.
166. Understanding, then, that the propagation of an excitation
depends on the state of tension of the tissue, and always follows
the line of least resistance, whichever that may be at the moment,
we have to inquire whether the transmission takes place only in one
direction, from periphery to centre in sensory nerves, and from centre
to periphery in motor nerves? By most physiologists this is answered
affirmatively. Indeed a special property has been assigned to each
nerve, in virtue of this imaginary limitation of centripetal and
centrifugal conduction. The “nerve-current” (accepted as a physical
fact, and not simply a metaphor) is supposed to “flow” from the
central cells along the motor nerve to the muscles; but by a strange
oversight the current is also made to “flow” _towards_ the central
cells which are said to produce it! Now although the fact may be, and
probably is, that normally the sensory nerve, being stimulated at its
peripheral end, propagates the stimulation towards the centre, and the
motor nerve propagates its central stimulation towards the periphery,
the question whether each nerve is not capable of transmission in
both directions is not thus answered. _A priori_ it is irrational to
assert that nerves fundamentally alike in composition and structure
are unlike in properties; and we might as well suppose that a train
of gunpowder could only be fired at one end, as to suppose that a
nerve could only be excited at one end. And how does the evidence
support this _a priori_ conclusion? Dubois Reymond proved that each
nerve conducted electricity in both directions; but as Neurility
has not been satisfactorily shown to be identical with the electric
current, this may not be considered decisive. Such a doubt does not
hang over the following facts. M. Paul Bert, pursuing John Hunter’s
curious experiments on animal _grafting_, has grafted the tail of
a rat under the skin of the rat’s back, the tip of the tail being
inserted under the skin, its base rising into the air, so that there
is here an inversion of the normal position. In the course of time
Sensibility gradually reappears in this grafted tail; and at the
end of about twelve months the rat not only feels when the tail is
pinched, but knows _where_ the irritation lies, and turns round to bite
the pincers.[186] Here we have a case of a sensory nerve reversed,
yet transmitting stimulation from the base to the tip of the tail,
instead of from the tip to the base, as in a normal organ. Vulpian
and Philippeaux having divided two nerves, united the central end of
the sensory nerve with the peripheral end of the motor nerve; when
the organic union was complete, and each nerve was formed out of the
halves of two different nerves, the effect of pinching one of these
was to produce simultaneously pain and movement, showing that the
excitation was transmitted upwards to the centre, and downwards to the
muscles.[187] It may be compared with a train of gunpowder having a
loaded cannon at one end and a bundle of straw at the other, when if a
spark be dropped anywhere on this train, the flame runs along in both
directions, explodes the cannon, and sets alight the straw.
167. Indeed we have only to remember the semi-liquid nature of the
axis cylinder to see at once that it must conduct a wave of motion as
readily in one direction as in another. A liquid transmits waves in any
direction according to the initial impulse. There is consequently no
reason for asserting that because the usual direction is centripetal
in a sensory nerve, and centrifugal in a motor nerve, each nerve is
_incapable_ of transmitting excitations in both directions. And I think
many phenomena are more intelligible on the assumption that neural
transmission is in both directions. If the eye is fixed steadfastly
on a particular color during some minutes, the retina becomes
exhausted, and no longer responds to the stimulus of that color: here
the stimulation is of course centripetal. But if instead of looking
intently on the color, the mind (in complete absence of light) pictures
it intently, _this cerebral image is equally capable of exhausting the
retina_; and unless we believe that color is a cerebral, not a retinal
phenomenon (which is my private opinion), we must accept this as proof
of a centrifugal excitation of a sensory tract. Another illustration
may be drawn from the muscular sense. There may be a few sensory fibres
distributed to muscles; but even if the observations of Sachs[188]
should be confirmed, I do not think that all muscle sensations can be
assigned to these fibres, but that the so-called motor fibres must
also co-operate. When a nerve acts upon a muscle, the muscle reacts
on the nerve; and when a nerve acts on a centre, the centre reacts on
the nerve. The agitation of the central tissue cannot leave the nerve
which blends with it unaffected; the agitation of the muscular tissue
must also by a reversal of the “current” affect its nerve. Laplace
points out how the movement of the hand which holds a suspended chain
is propagated along the chain to its terminus, and if when the chain
is at rest we once more set that terminus in motion, the vibration
will remount to the hand.[189] The contraction of a muscle will not
only stimulate the sensory fibres distributed through it, but also, I
conceive, stimulate the very motor fibres which caused the contraction,
since these fibres blend with the muscle.[190]
168. To understand this, it is necessary to remember that the
stimulation of a nerve does not arise[191] in the _changed state_ of
that nerve, but in the _process of change_, i. e. the disturbance of
the tension. The duration of the stimulation is that of the changing
process, and the intensity increases with the differential of the
velocity of change. So that when a nerve which has been excited by a
change of state returns to its former state, this return--being another
change--is a new excitation. That it is not the changed state, but the
change, which is operative, explains the fact noted by Brown Séquard:
a frog poisoned by strychnine, when decapitated and all respiration
destroyed, will remain motionless for days together, if carefully
protected from all external excitation; but its nervous system is in
such a state of tension all this time that the first touch produces
general convulsions. Freusberg also notes that if a brainless frog be
suspended by the lower jaw, and one foot be pinched, the other leg is
moved at first, then quickly droops again, and remains at rest until
the pincers are removed from the pinched foot, when suddenly all four
legs are violently moved by the _stimulation which the simple removal
produces_. Let us also add the well-known and significant fact that
if a nerve be divided rapidly by a sharp razor, neither sensation
nor motion is produced, because the intensity of a stimulus being,
to speak mathematically, _the function of the changing process_, the
duration of the process is in this case too brief. On the same ground
the application of a stimulus will excite no movement, if the force be
very slowly increased from zero to an intensity which will destroy the
nerve; but at any stage a _sudden_ increase will excite a movement.
169. We may group all the foregoing considerations in this formula:
LAW I. Every neural process is due to a sudden disturbance of the
molecular tension. The liberated energy is discharged along the
lines of least resistance.
The conditions which determine the lines of least resistance are
manifold and variable. The nervous system is a continuous whole, each
part of which is connected with diverse organs; but in spite of this
anatomical diversity, the deeper uniformity causes the activity of
each part to depend on and involve the activity of every other, more
or less. By “more or less” is meant, that although the excitation of
one part necessarily affects the state of all the others, because of
their structural community, so that _each sensation and each motion
really represents a change in the whole organism_, yet the responsive
discharge determined in each organ by this change, depends on the
tension of the organ and its centre at that moment. A bad harvest
really affects the whole nation; but its effect is conspicuous on the
welfare of the poor rather than of the rich, although the price of
bread is the same to rich and poor. Nervous centres, and muscular or
glandular organs, differ in their excitability; one condition of this
greater excitability being the greater frequency with which they are
called into activity. The medulla oblongata is normally more excitable
than the medulla spinalis; the heart more than the limbs. Hence a
stimulus which will increase the respiration and the pulse may have no
_appreciable_ effect on the limbs; but some effect it must have.
170. Imagine all the nerve-centres to be a connected group of bells
varying in size. Every agitation of the connecting wire will more or
less agitate all the bells; but since some are heavier than others,
and some of the cranks less movable, there will be many vibrations
of the wire which will cause some bells to sound, others simply to
oscillate without sounding, and others not sensibly to oscillate.
Even some of the lighter bells will not ring if any external pressure
arrests them; or if they are already ringing, the added impulses, not
being rhythmically timed, will _arrest_ the ringing. So the stimulus
of a sensory nerve agitates its centre, and through it the whole
system; usually the stimulation is mainly reflected on the group of
muscles innervated from that centre, because this is the readiest path
of discharge; but it sometimes does not mainly discharge along this
path, the line of least resistance lying in another direction; and the
discharge never takes this path without also irradiating upwards and
downwards through the central tissue. Thus irradiated, it falls into
the general stream of neural processes; and according to the state in
which the various centres are at the moment it modifies their activity.
A nervous shock--physical or mental--sensibly affects all the organs. A
severe wound paralyzes, for a time, parts far removed from the wounded
spot. A blow on the stomach will arrest the heart; a fright will do
the same. Terror relaxes the limbs, or sets them trembling; so does
a concussion: if a frog be thrown violently on the ground, all its
muscles are convulsed; but if the nerves of one limb be divided before
the shock, the muscles of that limb will not be convulsed.
171. We are apt to regard the discharge on the moving organs as if
that were the sole response of a stimulation; but although the most
conspicuous, it is by no means the most important effect. Besides
exciting the muscles, more or less, every neural process has its
influence on the organic processes of secretion, and effects thermal
and electrical changes. Schiff has demonstrated that every sensation
raises the temperature of the brain; Nothnagel, that irritation of a
sensory nerve causes constriction of the cerebral arteries, and hence
cerebral anæmia. Brown Séquard and Lombard find the temperature of
a limb raised when its skin is pinched, and lowered when the skin
_elsewhere_ is pinched. Georges Pouchet has shown that fishes change
color according to the brightness or darkness of the ground over which
they remain; and these changes are dependent on nervous stimulation,
mainly through the eye, division of the optic nerves preventing the
change. These are so many _a posteriori_ confirmations of what _a
priori_ may be foreseen. They are cited here merely to enforce the
consideration, seldom adequately kept before the mind, that every
neural process is a change which causes other changes in the whole
organism.
STIMULI.
172. Stimuli are classed as external and internal, or physical and
physiological. The one class comprises all the agencies in the
External Medium which _appreciably_ affect the organism; the other
class all the changes _in_ the organism which appreciably disturb the
equilibrium of any organ. Although the pressure of the atmosphere, for
example, unquestionably affects the organism, and determines organic
processes, it is not reckoned as a stimulus unless the effect become
appreciable under sudden variations of the pressure. In like manner
the blood is not reckoned among the internal stimuli, except when
sudden variations in its composition, or its circulation, determine
appreciable changes. Because the external stimuli, and the so-called
Senses which respond to them, are more conspicuous than the internal
stimuli and the Systemic Senses, they have unfortunately usurped too
much attention. The massive influence of the Systemic Sensations in
determining the desires, volitions, and conceptions of mankind has not
been adequately recognized. Yet every one knows the effect of impure
air, or a congested liver, in swaying the mental mood; and how a
heavy meal interferes with muscular and mental exertion.[192] What is
conspicuous in such marked effects, is less conspicuously, but not less
necessarily, present in slighter stimuli.
173. A constant pressure on the tympanum excites no sound; only a
rhythmic alternation of pressures will excite the sensation. A constant
temperature is not felt; only changes in temperature. If Light and
Sound were as uniform as the circulation of the blood, or the pressure
of the atmosphere, we should be seldom conscious of the existence of
these stimuli. But because the changes are varied and marked, our
attention is necessarily arrested by them. The changes going on within
the tissues are too graduated to fix the attention; it is only by
considering their cumulative effects that we become impressed with
their importance. For example, the development of the sexual glands
determines conspicuous physical and moral results--we note consequent
effects on voice, hair, horns, structure of the skull and size of the
muscles, no less than the rise of new feelings, desires, instincts,
ideas. Any organic interference with the activity of the ovaries will
alter the moral disposition of the animal: suppression of this organic
process means non-development of the feelings of maternity; the moral
superstructure is absent because its physical basis is wanting.
174. Blood supplies the tissues with their plasmodes; a constant
supply of oxygenated blood is therefore necessary to the vitality of
the tissues. But it is an error to suppose that oxygen is the special
stimulus of nerve-centres, or that their activity depends on their
oxidation; on the contrary, the deficiency of oxygen or surplus of
carbonic acid is that which stimulates. When saturated with oxygen,
the blood paralyzes respiration; when some of the oxygen is withdrawn,
respiration revives. Here--as in all other cases--we have to remember
that differences in degree readily pass into differences in kind,
so that an excess of a stimulus produces a reversal of the effect;
thus although surplus of carbonic acid excites respiratory movements,
excess of carbonic acid causes Asphyxia. Abundance of blood is
requisite for the continuous activity of nerve-centres; but while a
temporary deficiency of blood renders them more excitable, too great a
deficiency paralyzes them. Anæmia, which causes great excitability, and
convulsions (so that nerves when dying are most irritable), may easily
become the cause of the death of the tissue. There are substances which
can only be dissolved by a given quantity of liquid; if this quantity
be in excess, they are precipitated from the solution. There are
vibrations of a given order which cause each string to respond; change
the special order, and the string returns to its repose.
In the stillness and darkness of the night we are excluded from most
of the external stimuli, yet a massive stream of systemic sensations
keeps the sensitive mechanism active, and in sleep directs the dreams.
The cramps and epileptiform attacks which occur during sleep are most
probably due to the over-excitability produced by surplus carbonic
acid. To temporary anæmia may be assigned the strange exaggeration of
our sensations during the moments which precede awakening; and the
greater vividness of dream-images.
It is only needful to mention in passing the varied stimuli by which
cerebral changes act upon the organism. The mention of a name will
cause a blush, a brightening of the eye, a quickening of the pulse. The
thought of her absent infant will cause a flow of milk in the mother’s
breast.
175. We may formulate the foregoing considerations in another law:
LAW II. The neural excitation, which is itself a change, directly
causes a change in the organ innervated, and indirectly in the
whole organism.
The significance of this law is, that although for the convenience
of research and exposition we isolate one organ from the rest of the
organism, and one process from all the co-operant processes, we have to
remember that this is an artifice, and that in reality there is no such
separation.
STIMULATION.
176. Passing now from these general considerations to their special
application, we may formulate the law of stimulation:
LAW III. A faint or moderate stimulation increases the activity
of the organ; but beyond a certain limit, increase of
stimulation diminishes, and finally arrests, the activity.
Duration of stimulation is equivalent to increase.
A muscle stimulated contracts; if the stimulation be repeated,
the muscle becomes tetanized, and in this state has reached its
limit; a fresh stimulation then _relaxes_ the muscle. A very faint
stimulation of the vagus quickens the pulsation of the heart, but a
slight increase, or duration of the stimulation, slackens and arrests
the heart.[193] Every one knows how a moderate feeling of surprise,
pleasure, or pain quickens the heart and the respiration; and how a
shock of surprise, joy, grief, or great physical pain depresses,
and even arrests them. Excess of light is blinding; excess of sound
deafening.
177. The nervous system is incessantly stimulated, and variably.
Hence a great variation in the excitability of different parts. While
the regular and moderate activity of one part is accompanied by a
regular flow of blood to it, so that there is a tolerably constant
rhythm of nutrition and discharge, any irregular or excessive activity
exhausts it, until there has been a nutritive restoration. We can
thus understand how one centre may be temporarily exhausted while
a neighboring centre is vigorous. Cayrade decapitated a frog, and
suspended light weights to each of its hind legs; when either leg was
stimulated, the weight attached to it was raised. After each repetition
the weight was raised less and less, until finally the weight ceased to
be raised: the centre had been exhausted. But now when the other leg,
which had been in repose, was stimulated, it energetically contracted,
and raised its attached weight; showing that its centre was not
exhausted by the action of the other.[194]
178. This seems in contradiction with the principle that the excitation
of one centre is an excitation of all. It also seems in contradiction
with the principle urged by Herzen, that irritation of one sciatic
nerve _diminishes_ the excitability of the opposite leg; and this again
seems contradicted by the principle urged by Setschenow, that although
moderate excitation of one sciatic nerve will diminish the excitability
of the other, a powerful excitation will increase it.
179. All three principles are, I believe, exact expressions of
experimental evidence; and their seeming contradictions may be
reconciled on a wider survey of the laws of neural activity,
interpreted according to the special conditions of each case. These
laws may be conveniently classified as laws of Discharge, and Laws of
Arrest; the second being only a particular aspect of the first.
THE LAW OF DISCHARGE.
180. The physiological independence of organs, together with their
intimate dependence in the organism, and the fact that this organism is
incessantly stimulated from many sides at once, assure us _a priori_
that the “waves” of molecular movement due to each stimulus must
sometimes interfere and sometimes blend with others, thus diverting or
neutralizing the final discharge in the one case, and in the other case
swelling the current and increasing the energy of the discharge. We
are accustomed to speak of one part “playing on another,” sympathizing
with another, and so on; but what is the process expressed in these
metaphors? When an idea, or a painful sensation, quickens the pulse,
or increases the flow of a secretion, we are not to imagine that from
a spot in the cerebrum, or the surface, there is a nerve-fibre going
directly to the heart, or the gland, transmitting an impulse; in each
case the central tissue has been agitated by a sudden change at the
stimulated point, and the discharge on heart and gland is the resultant
of this agitation along the lines of least resistance. The nerves of
the great toe, for example, pass into the spinal cord at a considerable
distance from the spot where the nerves of the arm enter it; when,
therefore, the great toe is pinched, the arm does not move by direct
stimulation of its nerves, but by the indirect stimulation which has
traversed the whole central substance.
181. This is intelligible when we know that the whole central substance
is continuous throughout; but the difficulty arises when we have to
explain why, if this central substance is stimulated throughout,
_only_ arms and legs respond; in other words, why the toe-centre
“plays upon” the arm-centre, and not on the others? When a frog is
decapitated, if we gently touch one leg with the point of the scalpel,
the leg will move, but only this leg. Prick more forcibly, and both
legs will move. Keep on pricking, and all four legs are drawn up, and
the frog hops away. Each excitation was propagated along the cord;
but the discharge was restricted in the first case to one limb, in
the second to two, in the third it involved all the muscles of the
trunk. At the sight of a friend a dog wags his tail gently: as there
is no _direct_ connection between the optic nerves and the tail, this
playing of one centre on another must be by the agency of intermediate
centres; and we know that if the dog’s spinal cord be divided, this
excitation from the optic centre is no longer possible, yet the tail
will wag if the abdomen be tickled, or the leg pinched. Now compare the
effect on the dog produced by the sight of his master, or of a friend
accustomed to take him out. There is no longer a gentle wagging of the
tail, but an agitation of the whole body: he barks, leaps, and runs
about; the central stimulation is discharged through many outlets; and
could we test the effect, we should find an appreciable alteration
in the thermal and electrical condition of the whole organism, with
corresponding changes in circulation, secretion, etc. So different are
the consequences of two slightly different retinal impressions mingling
their stimulations with the same mass of central substance!
182. The discharge is determined by two conditions: the state of
tension, and the energy of the stimulation. _The state of tension is
increased by every stimulation which falls short of a discharge_; that
is to say, faint and frequent stimulation augments the excitability,
whereas powerful stimulation exhausts it. When, therefore, one
wave succeeds another in the same direction, it reaches a centre
more disposed to discharge; or, as Cayrade expresses it, “a certain
agitation of the cells is necessary for the manifestation of their
property of reaction, in the same way that the concentric circles
produced on the surface of water by a falling stone are more rapid and
more numerous if a stone has already agitated the surface.”
183. So much for the tension. What has been called the energy of the
stimulation is more complicated. It is not measurable as a simple
physical process; we cannot say that a given quantity of any external
force will determine a given discharge. It is mostly complicated by
psychical processes, and these so modify the result that instead of the
predicted discharge there is arrest, or discharge from another centre.
Press a dog’s skin with increasing violence, and the effect increases
from pleasurable to painful irritation; but whether the dog will cry
out and bite, or cry out and struggle to escape, depends upon whether
the pincher is a stranger or a friend. If you hurt a dog while removing
a thorn from its foot it will cry out, but although the pain causes it
to initiate a biting movement, by the time your hand is reached that
movement will have been changed, and the dog will lick the hand which
he knows is hurting him in the endeavor to relieve him of the thorn.
The co-operation of the mind is here evident enough. A purely psychical
process has interfered with the purely physiological process. And I
shall hereafter endeavor to show that psychical processes analogous in
kind though simpler in degree are really co-operant in actions of the
spinal cord. The dog would be said to discriminate between the pain
inflicted by a friend, and the same pain inflicted by a stranger. In
other words, the sensitive mechanism would be differently determined
in the direction of discharge, although the initial stimulation was the
same in each case. If we admit that the resulting action is in each
case the consequence of the particular group of elements co-operating,
there will be no ground for denying that analogous _discrimination_
is manifested by the brainless animal, who also responds differently
to different external stimuli, and differently to the same stimulus
under different central conditions. The brainless frog croaks if its
back be gently stroked with the handle of a scalpel; but if the point
be used, or if the handle be roughly pressed, instead of croaking,
the frog raises his leg in defence. Here the difference in the
peripheral irritation has excited a different reaction in the centre;
and this might be interpreted as purely physical; if now the leg be
fastened, and the movement of defence be thus prevented, the frog will
employ the other leg; or adopt some other means of relieving itself
from the irritation. It was a mass of registered experiences which
determined the dog not to bite his master. An analogous registration
of experiences determines the changed reactions of the brainless frog.
But this is a point which can only be touched on in passing here, and
it is touched on merely to facilitate our exposition of the complicated
conditions of neural discharge. These may be formulated in
184. LAW IV. The simultaneous influence of several stimuli, each
of which separately excites the same centre, is cumulative:
stimuli then assist each other, and their resultant is their
_arithmetical_ sum.
Simultaneous stimuli, each of which excites a different centre,
_interfere_ with each other’s energy, and their resultant is
their _algebraical_ sum.
In this law there is a condensed expression of that composition of
forces which may either result in Discharge or Arrest. By simultaneity
is not to be understood merely the coincidence of impressions, but also
the reverberations of impressions not yet neutralized by others. Thus
when Sensibility is tested by the now common method,[195] it is found
that if one leg is withdrawn after a lapse of, say, ten pendulum beats,
the other leg, which has not been irritated, will nevertheless, on
irritation, be withdrawn in less than ten beats, provided the central
agitation caused by the first stimulation has _not yet subsided_. But,
on the contrary, the withdrawal will be considerably deferred, or
even prevented altogether, if at the same time that the leg is acted
on by the acid, a more powerful excitation takes place in some other
part of the body. In the one experiment we see simultaneous excitation
in the same centre and the same direction. In the other simultaneous
excitation in different centres. The more powerful excitation
suppresses the discharge from the less powerful; but although it
prevails, it loses just as much force as it arrests.[196]
185. There is another very interesting experiment by Freusberg, which
must be cited here.[197] When the sciatic nerve is divided, the frog’s
leg is of course not withdrawn from the acidulated water, because in
that case no sensory excitation is propagated from the skin to the
centre; but although there is no stimulation from the skin, there is
one from the muscles, as appears in the fact that if a small weight be
suspended on this leg, the other leg is more rapidly withdrawn from the
acidulated water--the action of the muscles having affected the centre
and increased its excitability.
186. When the motor group of one leg is moderately stimulated, the
discharge is confined to the muscles of that one leg; and according
to Herzen the excitability of the motor group of the other leg is
thereby somewhat diminished. But if the stimulation be increased, there
is an irradiation to the other group, which irradiation, although
not sufficient to excite a discharge, renders it much more _ready_
to discharge, so that a feeble stimulus suffices. This accords with
Setschenow’s observations, and is confirmed by Freusberg’s experiment,
in which, when one leg was stimulated by acid, if the acid were not
wiped off but allowed to keep up the irritation, the other leg moved
without being irritated; and this other leg having come to rest, when
in its turn dipped in the acid, was more rapidly withdrawn than the
first leg had been on first being stimulated; showing that the central
groups had become more excitable by the stimulation of either leg.
187. While it is intelligible that an excitation of one group should
increase the activity of neighboring groups, by an increase of the
vascular activity of the region, it is not so readily intelligible why
the feebler excitation of one group should diminish the excitability of
its neighbor; yet the facts seem to warrant both statements.
188. The conditions which determine Discharge are obscure. We may,
however, say that anatomical and physiological data force the
conclusion that whenever the central tissue is powerfully stimulated
in any one part, there is either a discharge, or a greater tension
(tendency to discharge) in every other part; in consequence of
which, every fresh stimulus in the _same_ direction finds the parts
more prepared to react; while every fresh stimulus in a _contrary_
direction meets with a proportional resistance. Stated thus generally,
the principle is clear enough; but the immense complication of
stimulations, and the statical variableness of the organs, renders its
application to particular cases extremely obscure. Why does the ticking
of a clock arrest the attention, even with unpleasant obtrusiveness, at
one time, and presently afterwards cease to be heard at all? Why does
the cut of a knife cause intense pain, and a far greater cut received
during the heat and agitation of a quarrel pass unfelt? Why will the
same external force excite convulsions in all the muscles, and at
another time scarcely be distinguishable? These are consequences of the
temporary condition of the centres; but there are permanent conditions
which in some organisms determine equally variable results. Thus the
shock of terror which will simply agitate one person, will develop an
epileptic attack in another, and insanity in a third; just as exposure
to cold will in one person congest the liver, in another the lungs. A
loud and sudden sound causes winking in most persons, and in many a
sort of convulsive shock. The harsh noise of a file causes a shiver in
some persons, and in others “sets the teeth on edge,” while in others
it causes an increased flow of saliva.
189. Nerves and centres have different degrees of excitability. The
nerve-terminals in the skin are more sensitive to impressions than
those in the mucous membrane; those in the alimentary canal are more
sensitive than those in the peritoneum; and all nerve-terminals are
more sensitive than nerve-trunks. A touch on the surface of the larynx
will produce a cough, but the nerve-trunk itself may be pinched or
galvanized without producing any such reflex. Moreover, there is the
difference of grouping. If the skin of the abdomen be tickled, there
is a reflex on the adductor and extensor muscles of the leg; but these
movements are reversed if the skin of the back be tickled. Nor indeed
are these movements invariable in either case; the one series will
sometimes quite suddenly change to the other, if the irritation is kept
up. That one and the same stimulus applied to the same spot should now
excite this group and now the other, shows that _both_ motor groups
are affected, and that the discharge takes place from the one which at
the time being is in the highest tension. The alternation of tension
explains rhythmical discharge.
THE LAW OF ARREST.
190. The Law of Arrest is only another aspect of the Law of Discharge,
and may be regarded as the conflict of excitations. If a stranger
enters the room where a woman lies in labor, there will often be caused
a sudden cessation of the uterine contractions.[198] Again, every one
knows how the breathing and the beating of the heart are arrested by
the idea of danger. The arrest is in each of the three cases only
temporary, because when the shock of the new stimulus has caused its
discharge (arrest), the peripheral irritation which caused the former
discharges resumes its influence, and uterus, heart, and diaphragm
begin to move again, even more energetically. Note, moreover, that not
only will the cerebral excitation arrest the spinal discharge--an idea
check the contractions of the uterus or the heart--but the reverse
also takes place. The brain of the woman may be intently occupied with
some scheme for the education or welfare of her expected child, but no
sooner do the labor pains set in, than all these cerebral combinations
are arrested.
191. One sensation arrests another; one idea displaces another. If
the foreleg of a headless frog be irritated, the hind-leg will also
be moved by the stimulation; or _vice versa_. Here there has been a
propagation of the excitation in either direction. But if while the
legs are thus irritated, and the centres are ready to discharge,
another and more powerful irritation reach the centre--say by pinching
the skin of the back--there will be no discharge on the legs. If the
vagus be irritated, the heart is arrested; but this does not take place
if at the same time, or immediately before, the foot has been sharply
pinched. A few gentle taps on the abdomen suffice to stop the heart;
but if a drop of acid be previously placed on the skin, we tap in vain,
the heart continues to beat. Brown Séquard cites several cases in which
convulsions were arrested by irritation of sensitive surfaces;[199]
and Dr. Crichton Browne records a case of a patient in whom there was
abolition of spinal reflex, due to cerebral irritation: tickling the
soles of the feet, or pricking the toes, which normally excites reflex
movements, in this case excited none whatever. “This seems to prove
that nerve currents, set in motion by irritation of the brain, or some
of its convolutions, transmitted down the cord, may inhibit reflex
action.”[200] Examples might indefinitely be multiplied. Pinch the
skin of a rabbit between the eyes, and you will observe that pulse and
respiration are slackened; but if the tail, which is very sensitive,
be pinched, this slackening is only momentary, and is succeeded by a
quickening--unless the pain be great. Even the effect of intense pain
may be neutralized by stimulating the vagus--just as the effect of
stimulating the vagus may be neutralized by pain. Claude Bernard found
that having dropped ammonia on the eyelid of a dog, the pain caused a
convulsive closure of the lid; but on galvanizing the vagus, the lid
opened again, to be closed when the galvanism ceased.[201] When the
heart is beating faintly (as in syncope), any irritating vapor applied
to the nostrils will cause a more energetic pulsation; yet a very
irritating vapor lowers the action of the heart beating normally, and
will even arrest that of a rabbit. Over-stimulation has almost always
the opposite effect of moderate stimulation.
192. While there seems every reason to believe that an excitation
necessarily affects the whole cerebro-spinal axis, there is no doubt
that there is a certain restriction of this irradiation to definite
paths, i. e. the responsive discharge is confined to definite groups.
Some of these restrictions are connate pathways: we bring them with us
at birth; but most of them are pathways acquired after birth. The boy
who sheds tears at parting from his mother when he goes to school, will
shed no tears when he parts from her to go to college, nay, perhaps
will shed none when he parts from her forever: not that his love has
lessened, but that the idea of such expression of it as “unmanly” has
become an organized tendency and arrests the tears. A youth of southern
race, who has not learned to be ashamed of tears, weeps freely under
such circumstances.
193. The pathways organized at birth are not many. Examples are the
inspiration which follows expiration; the movements of coughing when
the larynx is tickled; the movements of swallowing, sneezing, etc. Even
these may be arrested for a brief time by what is called “the will”;
but when once the discharge begins in any part of the mechanism, the
whole group is necessarily involved and the action is then inevitable.
Many of the reflex actions which are universal are nevertheless
acquired. Winking, for instance, when an object approaches the eye, is
universal among us, but is never seen in infants, nor in animals. It
is even doubtful whether the drawing up of the leg when the toes are
pinched is not an acquired reflex. Doubtful, I mean, in this sense,
that although the fact of non-withdrawal is observable in infants, who
cannot localize their sensations, this may be due to the imperfect
development of their nervous system. Mr. Spalding has proved that
although the callow bird cannot fly, the mechanism of flight is no
sooner developed than the action follows at once, without any previous
tentative experiences.
194. By experience we learn to restrict the paths of irradiation,
so as to wink with one eye while the other is unmoved, to bend one
finger while the rest are extended, to move one limb, or one group of
muscles, while the others are at rest; in short, to execute any one
particular action, and not at the same time agitate superfluously many
other organs. The boy when first learning to write is unable to prevent
the simultaneous motions of tongue and legs, which are ludicrously
irrelevant to the purpose of writing; but he learns to keep all his
organs in subjection, and only the eyes and hands active.[202] An
analogous restriction takes place in thinking. A train of thought is
kept up by the exclusion of all suggestions which are not pertinent;
and the power of the thinker is precisely this power of concentration.
THE HYPOTHESIS OF INHIBITORY CENTRES.
195. The facts and their formulated laws which have just been adduced
furnish a sufficient explanation of all the phenomena of arrest
which of late years have been detached and assigned to a special
mechanism of inhibitory nerves and centres. In spite of the eminent
authorities countenancing the hypothesis of a particular set of
inhibitory nerves, and particular centres of inhibition, I must confess
that the hypothesis appears to me inadmissible; and that I side with
those physiologists who hold that each nerve and each centre has its
inhibitory action. Indeed, if the action of arrest be, as I maintain,
only another aspect of the action of discharge, the result of the
conflict of forces, to say that all centres have the property of
excitation, is to say that all have the properties of discharge and
arrest: the discharge is only the resultant of the conflict along the
line of least resistance; the arrest is the effect of the conflict
along the line of greatest resistance. The observed phenomena of
arrest are so varied and numerous that the upholders of the inhibitory
hypothesis have been forced to invent not only arresting centres,
but centres which arrest these arresting centres! Dr. Lauder Brunton
candidly remarks: “At present our notions of nervous action seem to be
getting as involved as the Ptolemaic system of astronomy, and just as
epicycles became heaped upon cycles, so nerve-centres are being added
to nerve-centres. And yet, clumsy though the system may be, it serves
at present a useful purpose, and may give real aid until a better
is discovered.” I do not think a Copernicus is needed to discover
a better. The Law of Arrest as a general neural law suffices, when
the right conception of a _centre_ as a physiological rather than an
anatomical designation is admitted. (See p. 173.)
196. It would be out of place here to consider the conflicting evidence
which at present renders the question of the movements of the heart
one of the most unsatisfactory in the whole range of experimental
physiology. After devoting much time to it, and after writing a long
chapter on it, I suppress what I had written, and content myself with
the statement that no advantage whatever is derived from the hypothesis
of a special mechanism of arrest, unless perhaps in giving a temporary
precision to the direction of research. I mean that the search for
special centres may lead to the discovery of the particular paths to
which an impulse is restricted in any one action: as, for instance,
the vagus in retarding the pulsation of the heart. If the cerebrum can
determine a movement, and combine various movements, it is a centre of
arrest; if the cerebellum can determine and regulate movements, it is a
centre of arrest; if the medulla oblongata can determine and regulate
movements, it is a centre of arrest; if the medulla spinalis can
determine and combine movements, it is a centre of arrest; if a nerve
can dilate a constricted blood-vessel, or constrict a dilated one, it
is a nerve of arrest. In other words, every centre exerts its action
either in discharging, or in arresting the discharge of some other
centre.
The physiological process of Arrest may be physically interpreted
as Interference;[203] not that the process in nerve-tissue is to be
understood as the _same_ as that observed in fluids, or that the
metaphor of neural waves is to be taken for more than an intelligible
picturing of the process; the difference in the two agents forbids our
admitting the resemblance to be more than analogical. Thus interpreted,
however, we see that not only will one centre arrest the action of
another, but one nerve may be made to arrest itself! I mean that, under
similar conditions of interference, the stimulation which normally
follows on external stimulus may be inhibited by a previous, or a
counter stimulation. Thus the nerve which will be stimulated by a
chemical or mechanical stimulus, wholly fails to react if a constant
current is passing through it, although this constant current does not
itself cause a constant contraction. Remove the electrodes, and then
the chemical or mechanical stimulus takes effect. Or the experiment
may be reversed: let the nerve be placed in a saline solution, and
the muscles will be at once thrown into violent contraction; if the
electrodes are now applied to the nerve, the contractions suddenly
cease, to begin again directly the electrodes are removed.
ANATOMICAL INTERPRETATION OF THE LAWS.
197. The problem for the anatomist is twofold: First, given the
organ, he has to determine its function, or _vice versa_, given the
part of an organ, to determine its functional relation; secondly,
given the function, he has to determine its organ. The structural
and functional relations of nerves and centres have been ascertained
in a general way; we are quite sure that the posterior nerves carry
excitations from sensitive surfaces, that the anterior nerves carry
excitations to muscles and glands; and that the central gray substance
not only reflects a sensory excitation as a motor excitation, but
propagates an excitation along the whole cerebro-spinal axis. But when
we come to a more minute analysis of the functional activities, and
endeavor to assign their respective values to each part of the organic
mechanism, the excessive complexity and delicacy of the mechanism
baffles research. We are forced to grope our way; and the light of the
hypothetic lamps which we hold aloft as often misdirects as helps us.
The imaginary anatomy which at present gains acceptance, no doubt seems
to simplify explanations; but this seeming turns out to be illusory
when closely examined. The imagined arrangement of fibres and cells
we have seen to be not in agreement with observation; and were it
demonstrable, it would not account for the laws of propagation. Suppose
sensory fibres to terminate in cells, and fibres from these to pass
_upwards_ to other sensory cells and _transversely_ to motor cells, how
in such a connected system could irradiations take place, if the law of
isolated conduction were true? And how could isolated conduction take
place, if the excitation of a part were necessarily the excitation of
the whole? Why, for example, is pain not always irradiated? Why is it
even localized in particular spots, determining movements in particular
muscles; and when irradiation takes place, why is it circumscribed,
or--and this is very noteworthy--manifested in two widely different
places, the intercostal and trigeminal nerves? Why does the irritation
of intestinal worms manifest itself now by troubles of vision, now by
noises in the ear, and now by convulsions?
198. Answers to such questions must be sought elsewhere. Our first
search should be directed to the anatomical data, which have hitherto
been so imprudently disregarded. Under the guidance of the laws
formulated in this chapter, let us accept the anatomical fact of a
vast network forming the ground-substance in which cells and fibres
are embedded, and with which they are continuous; let us accept the
physiological principle Of similarity of property with similarity
of composition and structure; let us accept the hypothesis that the
discharge of neural energy is dependent on the degree of stimulus
and the degree of tension at the time being--and we shall have at
least a general theory of the process, though there will still remain
great obscurities in particular applications. We shall have before
us a vast network of pathways, all equally capable of conducting an
excitation, but not all equally and at all moments open. It will always
be difficult to determine what are the conditions which at any moment
favor or obstruct particular openings. Paths that have been frequently
traversed will of course be more readily traversed again; but this very
facility will _sometimes_ be an obstacle, since it will have caused
that path to be preoccupied, or have fatigued the organ to which it
leads.
199. Since the escape of an excitation must always be along the lines
of least resistance, an obvious explanation of the restriction to
certain paths has been to assume that some fibres and cells have
naturally greater resistance than others. But this explanation is
simply a restatement of the fact in other words. What is this greater
resistance? Why is it present in one fibre rather than in another?
We should first have to settle whether the resistance was in the
nervous pathway itself, or in the centre, or in the organ innervated;
an excitation might pass along the nervous tract, yet fail to change
the state of the centre, or the organ, sufficiently to produce an
appreciable response; and only those parts where an appreciable
response was produced would then be considered as having had the
pathways of propagation open.
200. When we reflect on the innumerable stimulations to which the
organism is subjected from so many various points, and remember
further that _each stimulation leaves behind it a tremor which
does not immediately subside_, we shall conceive something of the
excessive complexity of the mechanism, and marvel how any order is
established in the chaos. What we must firmly establish in our minds
is that the mechanism is essentially a _fluctuating_ one, its elements
being combined, recombined, and resolved under infinite variations
of stimulation. If it were a mechanism of fixed relations, such as
we find in machines, or in the “mechanism of the heavens,” we might
accept the notion of certain organites having greater resistance as
a consequence of their structure, just as one muscle resists being
moved by the impulse which will move another. Nor is it doubtful
that such differences exist in nervous organites; but the laws of
central excitation are not interpretable by any such hypothesis,
since we know that the paths which were closed against an impulse of
considerable energy may be all open to an impulse of feebler energy,
and that a slight variation in the stimulus will be followed by a wide
irradiation. For example, a grain or two of snuff will excite the
violent and complex act of sneezing, but the nerves of the nasal cavity
may be pinched, cut, or rubbed, without producing any such result.
One group of nervous organites will fail to involve the activity of
neighboring groups; and the simple movement of a single organ is then
all that appreciably follows the stimulation; yet by a slight change in
the stimulation, the organites are somewhat differently grouped, and
the result is a complex movement of many organs. It is this fluctuation
of combination in the organites which renders education and progress
possible. Those combinations which have very frequently been repeated
acquire at last an automatic certainty.
* * * * *
We are now in a position to examine with more precision the extremely
important laws of nervous action which are involved in the phenomena
designated by the terms Reflex Action, Automatic Action, and Voluntary
Action.
PROBLEM III.
ANIMAL AUTOMATISM.
“L’organisme le plus complexe est un vaste mécanisme qui résulte de
l’assemblage de mécanismes secondaires.”--CLAUDE BERNARD.
“Les corps vivants sont machines à l’infini.”--LEIBNITZ.
“Noi lamentiamo con Majendie che nel linguaggio fisiologico siensi
intruse le preopinioni psicologiche col trascico inevitabile
del vocaboli, ai quali codeste preopinioni si trovano legate.
Probabilmente questa fu una delle principali cagioni degli errori e
degli equivoci anatomofisiologici, da cui non poterono svincolarsi,
a loro insaputa, i cultori sperimentali della scienza, perchè nell’
interpretare i fenomeni osservati erano obbligati ad usare il
linguaggio di una false moneta in corso.”--LUSSANA _e_ LEMOIGNE,
_Fisiologia dei Centri Nervosi_, 1871, I. 16.
ANIMAL AUTOMATISM.
CHAPTER I.
THE COURSE OF MODERN THOUGHT.
1. Modern Philosophy has moved along two increasingly divergent lines.
One, traversed by Galileo, Descartes, Newton, and Laplace, had for
its goal the absolute disengagement of the physical from the mental,
i. e. the objective from the subjective aspect of phenomena, so
that the physical universe, thus freed from all the complexities of
Feeling, might be interpreted in mechanical terms. As a preliminary
simplification of the problem this was indispensable; only by it
could the First Notion of primitive speculation be replaced by the
Theoretic Conception of scientific speculation.[204] The early
thinker inevitably invested all external objects with properties and
qualities similar to those he assigned to human beings, and their
actions he assigned to human motives. Sun, moon, and stars seemed
living beings; flames, streams, and winds were supposed to be moved
by feelings such as those known to move animals and men. Nor was any
other conception then possible: men could only interpret the unknown
by the known, and their standard of all action was necessarily drawn
from their own actions. Not having analyzed Volition and Emotion,
above all not having localized these in a neuro-muscular system, men
could not suspect that the movements of planets and plants, and of
streams and stones, had motors of a different kind from the movements
of animals. The scientific conception of inert insensible Matter was
only attained through a long education in abstraction; and is assuredly
never attained by animals, or by savages. But no sooner were vital
conditions recognized, than the difference between vital and mechanical
movements emerged. When men learned that many of their own actions
were unaccompanied either by Love or Hate, by Pleasure or Pain, and
that many were unprompted by conscious intention, while others were
unaccompanied by conscious sensation, they easily concluded that
wherever the special conditions of Feeling were absent, the actions
must have some other motors. Intelligence, Emotion, Volition, and
Sensation being one by one stripped away from all but a particular
class of bodies, nothing remained for the other bodies but insensible
Matter and Motion. This was the Theoretic Conception which science
substituted for the First Notion. It was aided by the observation
of the misleading tendency of interpreting physical phenomena by
the human standard, substituting our fancies in the place of facts,
manipulating the order of the universe according to our imagination
of what it might be, or ought to be. Hence the vigilance of the new
school in suppressing everything pertaining to the subjective aspect
of phenomena, and the insistance on a purely objective classification,
so that by this means we might attain to a knowledge of things as they
are. By thus withdrawing Life and Mind from Nature, and regarding
the universe solely in the light of Motion and the laws of Motion,
two great scientific ends were furthered, namely, a classification
of conceptions, and a precision of terms. Objective phenomena made a
class apart, and the great aim of research was to find a mathematical
expression for all varieties under this class. Masses were conceived
as aggregates of Atoms, and these were reduced to mathematical points.
Forces were only different modes of Motion. All the numberless
differences which perception recognized as _qualities_ in things, were
reduced to mere variations in _quantity_. Thus all that was particular
and concrete became resolved by analysis into what was general and
abstract. The Cosmos then only presented a problem of Mechanics.
2. During this evolution, the old Dualism (which conceived a material
universe sharply demarcated from the mental universe) kept its
ground, and attained even greater precision. The logical distinction
between Matter and Mind was accepted as an essential distinction,
i. e. representing distinct reals. There was on the one side a group
of phenomena, Matter and Force; on the other side an unallied group,
Feeling and Thought: between them an impassable gulf. How the two
were brought into relation, each acting and reacting on the other,
was dismissed as an “insoluble mystery”--or relegated to Metaphysics
for such minds as chose to puzzle over questions not amenable to
experiment. Physics, confident in the possession of mathematical
and experimental methods which yielded definite answers to properly
restricted questions, peremptorily refused to listen to any suggestion
of the kind. And the career of Physics was so triumphant that success
seemed to justify its indifference.
3. In our own day this analytical school has begun to extend its
methods even to the mental group. Having reduced all the objective
group to mathematical treatment, it now tries to bring the subjective
group also within its range. Not only has there been more than one
attempt at a mathematical Psychology; but also attempts to reduce
Sensibility, in its subjective no less than in its objective aspect, to
molecular movement. Here also the facts of Quality are translated into
facts of Quantity; and all diversities of Feeling are interpreted as
simply quantitative differences.
4. Thus far the one school. But while this Theoretic Conception
stripped Nature of consciousness, motive, and passion, rendering it a
mere aggregate of mathematical relations, a critical process was going
on, which, analyzing the nature of Perception, was rapidly moving
towards another goal. Locke, Berkeley, Hume, and Kant, directing their
analysis exclusively to the subjective aspect of phenomena, soon broke
down the barriers between the physical and mental, and gradually merged
the former in the latter. Matter and its qualities, hitherto accepted
as independent realities, existing where no Mind perceived them, were
now viewed as the creations of Mind--their existence was limited to a
state of the percipient. The old Dualism was replaced by Idealism. The
Cosmos, instead of presenting a problem of Mechanics, now presented a
problem of Psychology. Beginning with what are called the secondary
qualities of Matter, the psychological analysis resolved these into
modes of Feeling. “The heat which the vulgar imagine to be in the fire
and the color they imagine in the rose are not there at all, but are in
us--mere states of our organism.” Having gained this standing-place,
there was no difficulty in extending the view from the secondary to the
primary qualities. These also were perceptions, and only existed in
the percipient. Nothing then remained of Matter save the hypothetical
unknown _x_--the postulate of speculation. Kant seemed forever to have
closed the door against the real Cosmos when he transformed it into
a group of mental forms--Time, Space, Causality, Quantity, etc. He
propounded what may be called a theory of mental Dioptrics whereby a
pictured universe became possible, as Experience by its own _a priori_
laws moulded _itself_ into a consistent group of appearances, which
produced the illusion of being a group of realities. He admitted,
indeed, that by the operation of Causality we are compelled to believe
in a Real underlying the appearances; but the very fact that this
Causality is a _subjective law_, is proof, he said, of its not being
an _objective truth_. Thus the aim of the mechanical conception was
to free research from the misleading complexities of subjective
adulterations, and view _things as they are_ apart from their
_appearances_; but this aim seemed illusory when Psychology showed that
Time, Space, Matter, and Motion were themselves not objective reals
except in so far as they represented subjective necessities; and that,
in short, things _are_ just what they _appear_, since it is only in the
relation of external reals to internal feelings that objects exist for
us.
5. Idealism has been the outcome of the psychological method. It
has been of immense service in rectifying the dualistic conception,
and in correcting the mechanical conception. It has restored the
subjective factor, which the mechanical conception had eliminated.
It has brought into incomparable clearness the fundamental fact that
all our knowledge _springs from_, and is _limited by_, Feeling. It
has shown that the universe represented in that knowledge, can only
be a picture of the system of things as these exist in relation to
our Sensibility. But equally with the mechanical conception it has
erred by incomplete analysis. For a complete theory of the universe,
or of any one phenomenon, those elementary conditions which analysis
has provisionally set aside must finally be restored. When Quality
is replaced by Quantity, this is an artifice of method, which does
not really correspond with fact. The quality is the fact given in
feeling, which we analytically refer to quantitative differences,
but which can never be wholly resolved into them, since it must be
presupposed throughout. One color, for example, may be distinguished
from another as having more or fewer undulations; and so we may by
abstraction, letting drop all qualitative characters, make a scale
of undulations to represent the scale of colors. But this is an
ideal figment. It is the representation of one series of feelings by
another series of different feelings. No variation of undulations will
really correspond with variation in color, unless we reintroduce the
suppressed _quality_ which runs through all color. Attempt to make one
born blind feel, or even understand, Color by describing to him the
kind of wave-movement which it is said to be, and the vanity of the
effort will be manifest. Movement he knows, and varieties of movement
as given in _tactile and muscular sensations_; but no combination and
manipulation of such experiences can give him the specific sensation
of Color. That is a purely subjective state, which he is incapable of
experiencing, simply because one of the essential factors is absent.
One set of objective conditions is present, but the other set (his
sense-organ) is defective. Without the “greeting of the spirit”
undulations cannot become colors (nor even undulations, for these
also are forms of feeling). Besides the sense-organ there is needed
the feeling of Difference, which is itself the product of past and
present feelings. The reproduction of other colors, or other shades of
color, is necessary to this perception of difference; and this involves
the element of Likeness and Unlikeness between what is produced and
reproduced. So that a certain mental co-operation is requisite even for
the simplest perception of quality. In fact, psychological analysis
shows that even Motion and Quantity, the two objective terms to which
subjective Quality is reduced, are themselves Fundamental Signatures
of Feeling;[205] so that here, as elsewhere, it is only by analytical
artifice that the objective can be divorced from the subjective. Matter
_is_ for us the Felt; its Qualities are differences of Feeling.
6. Not that this result is to be interpreted as freeing our Theoretic
Conception from its objective side, and landing us in Idealism,
which suppresses the real universe. The denial of all reality apart
from our minds, is a twofold mistake: it confounds the conception of
general relations with particular relations, declaring that because
the External in its relation to the sentient organism can only be
what it is felt to be, therefore it can have no _other_ relations to
other individual reals. This is the first mistake. The second is the
disregard of the constant presence of the objective real in every fact
of Feeling: the Not-Self is emphatically present in every consciousness
of Self.
The legitimate conclusion is neither that of Dualism nor of Idealism,
but what I have named Reasoned Realism (_Problems_, Vol. I. p. 201),
which reconciles Common Sense with Speculative Logic, by showing that
although the _truth_ of things (their _Wahrheit_) is just what we
perceive in them (our _Wahrnehmung_), yet their _reality_ is this, and
much more than this. _Things_ are what they are felt to be; and what
they are thought to be, when thoughts are symbols of the perceptions.
Idealism declares that they are _nothing but_ this. It is against this
_nothing but_ that Common Sense protests; and the protest is justified
by Reasoned Realism, which, taking a comprehensive survey of the facts,
thus answers the idealist: “Your synthesis is imperfect, since it
does not include _all_ the data--notably it excludes the fact of an
objective or Not-Self element in every feeling. You may, conceivably,
regard the whole universe as nothing but a series of changes in your
consciousness; but you cannot hope to convince me that I myself am
simply a change in yourself, or that my body is only a fleeting image
in your mind. Hence although I conclude that the Not-Self is to you, as
to me, undivorceable from Self, inalienable from Feeling, in so far as
it is felt, yet there must nevertheless be for both of us an existence
not wholly coextensive with our own. _My_ world may be my picture of
it; _your_ world may be your picture of it; but there is something
common to both which is more than either--an existent which has
different relations to each. _You_ are not _me_, nor is the pictured
Cosmos _me_, although I picture it. Looking at you and it, I see a vast
whole of which you are a small part; and such a part I conclude myself
to be. It is at once a picture and the pictured; at once subjective and
objective. To me all your modes of existence are objective aspects,
which, drawing from my own experience, I believe to have corresponding
subjective aspects; so that your emotions, which to me are purely
physical facts, are to you purely mental facts. And psychological
analysis assures me that all _physical facts are mental facts expressed
in objective terms_, and _mental facts are physical facts expressed in
subjective terms_.”
7. But while Philosophy thus replaces the conceptions of Dualism and
Idealism by the conception of the Two-fold Aspect, the special sciences
in their analytical career have disregarded the problem altogether.
The mechanical theory of the universe not only simplified research by
confining itself solely to the objective aspect of phenomena, but by
a further simplification set aside all vital and chemical relations,
to deal exclusively with mechanical relations. In ascertaining the
mathematical relations of the planetary system, no elucidation could
possibly be gained from biological or chemical conceptions; the
planets therefore were provisionally stripped of everything not
mechanical. In systematizing the laws of motion, it was necessary to
disengage the abstract relations from everything in any way resembling
spontaneity, or extra-mechanical agency: Matter was therefore, by
a bold fiction, declared to be inert, and its Motion regarded as
something superadded from without.
7_a_. And this was indispensable for the construction of those ideal
laws which are the objects of scientific research. Science, as we often
say, is the systematization of Experience under the forms of ideal
constructions. Experience implies Feeling, and certain fundamental
Signatures, all reducible to the primary discernment of Likeness and
Unlikeness. Hence Science is first a _classification_ of qualities
or discerned likenesses and differences; next a _measurement_
of quantities of discerned likenesses and differences. Although
measurement is itself a species of classification, it is distinguished
by the adoption of a standard unit of comparison, which, being precise
and unvarying, enables us to express the comparisons in precise and
unvarying symbols. Whether the unit of length adopted be an inch, a
foot, a yard, a mile, the distance of the earth from the sun, or the
distances of the fixed stars, the quantities thus measured are symbols
admitting of one invariable interpretation. The exactness of the
mathematical sciences is just this precision and invariability of their
symbols, and is not, as commonly supposed, the source of any superior
certainty as to the facts. The classificatory sciences, which deal with
qualities rather than with quantities, may be equally _certain_, and
represent fuller _knowledge_, because involving more varied feelings,
but they cannot pretend to exactness. Even on the quantitative side,
certainty is not identical with exactness. I may be quite certain that
one block of marble is larger than another--meaning that it affects
me more voluminously--but I cannot know how much larger it is, without
interpreting my feelings by the standard of quantity--the how-muchness
as represented by that standard. The immense advantages of exact
measurement need not be insisted on. The Biological Sciences, which
are predominantly classificatory, can never rival the Cosmological
Sciences in exactness; but they may reach a fuller knowledge; and
their certainty will assume more and more the character of exactness
as methods of measurement are applied to their classifications of
qualities. The qualitative and quantitative aspects of phenomena are
handled by the two great instruments, Logic and Mathematics, the second
being only a special form of the first. These determine the general
conceptions which are derived from our perceptions, and the whole
constitute Experience.
8. What is the conclusion to which these considerations lead? It
is that the separation of the quantitative from the qualitative
aspect of phenomena--the objective mechanical from the subjective
psychological--is a logical artifice indispensable to research; but
it is only an artifice.[206] In pursuance of this artifice, each
special science must be regarded as the search after special analytical
results; and meanwhile this method should be respected, and no
confusion of the boundaries between one science and another should be
suffered. Mechanical problems must not be confused by the introduction
of biological relations. Biological problems must not be restricted
to mechanical relations. I do not mean that the mechanical relations
present in biological phenomena are not to be sought, and, when found,
to be expressed in mechanical terms; I mean that such an inquiry must
be strictly limited to mechanical relations. Subjective relations
are not to be denied, because they are provisionally set aside, in an
inquiry into objective relations; but we must carefully distinguish
which of the two orders we are treating of, and express each in its
appropriate terms. This is constantly neglected. For example, nothing
is more common than to meet such a phrase as this: “A _sensory
impression_ is transmitted as a _wave_ of _motion_ to the brain, and
there being transformed into a state of _consciousness_, is again
reflected as a _motor_ impulse.”
The several sciences having attained certain analytical results, it
remains for Philosophy to co-ordinate these into a doctrine which will
furnish general conceptions of the World, Man, and Society. On the
analytical side a mechanical theory of the universe might be perfected,
but it would still only be a theory of mechanical relations, leaving
all other relations to be expressed in other terms. We cannot accept
the statement of Descartes that Nature is a vast mechanism, and Science
the universal application of mathematics. The equation of a sphere,
however valuable from a geometrical point of view, is useless as an
explanation of the nature and properties of the spherical body in
other relations. And so a complete theory of the mechanical relations
of the organism, however valuable in itself, would be worthless in
the solution of a biological problem, unless supplemented by all that
mechanical terms are incompetent to express.
9. The course of biological speculation has been similar to the
cosmological. It also began with a First Notion, which compendiously
expressed the facts of Experience. Nor can any Theoretic Conception be
finally adopted which does away with these facts, known with positive
certainty, and popularly expressed in the phrase: “I have a body, and
a soul.” We may alter the phrase either into “I _am_ a body, and I
_am_ a soul”; or into, “My body is only the manifestation of my soul”;
or, “My soul is only a function of my body”; but the fundamental
experiences which are thus expressed are of absolute authority, no
matter how they may be interpreted. That I have a body, or am a body,
is not to be speculatively argued away. That I move my arm to strike
the man who has offended me, or stretch out my hand to seize the fruit
which I see, is unquestionable; that these movements are determined by
these feelings, and are never thus effected unless thus determined, is
also unquestionable. Here are two sets of phenomena, having well-marked
differences of aspect; and they are grouped respectively under two
general heads, Life and Mind. Life is assigned to the physical
organism, or Body--all its phenomena are objective. Mind is assigned
to the psychical organism, or Soul--all its phenomena are subjective.
Although what is called my Body is shown to be a group of qualities
which are feelings--its color, form, solidity, position, motion--all
its physical attributes being what is felt by us in consequence of
the laws of our organization; yet inasmuch as these feelings have the
characteristic marks of objectivity, and are thereby referred to some
objective existence, we draw a broad line of demarcation between them
and other feelings having the characteristic marks of subjectivity,
and referring to ourselves as subjects. Psychological analysis shows
us that this line of demarcation is artificial, only representing a
diversity of aspect; but as such it is indispensable to science. We
cannot really separate in a sensation what is objective from what
is subjective, and say how much belongs to the Cosmos apart from
Sensibility, and how much to the subject pure and simple; we can only
view the sensation alternately in its objective and subjective aspects.
What belongs to extra-mental existence in the phenomenon of Color,
and what to the “greeting of the spirit,” is utterly beyond human
knowledge: for the ethereal undulations which physicists presuppose as
the cosmic condition are themselves subjected to this same greeting of
the spirit: they too are ideal forms of sensible experiences.
10. This conclusion, however, was very slowly reached. The distinction
of aspects was made the ground of a corresponding distinction in
agencies. Each group was personified and isolated. The one group was
personified in Spirit--an existent in every respect opposed to Matter,
which was the existent represented in the other group. One was said
to be simple, indestructible; the other compound, destructible. One
was invisible, impalpable, beyond the grasp of Sense; the other was
visible, tangible, sensible. One was of heaven, the other of earth.
Thus a biological Dualism, analogous to the cosmological, replaced
the First Notion. It was undermined by advances in two directions.
Psychology began to disclose that our conception of Matter was,
to say the least, _saturated_ with Mind, its Atoms confessedly
being ideal figments; and that all the terms by which we expressed
_material qualities_ were terms which expressed _modes of Feeling_;
so that whatever remained over and above this was the unknown _x_,
which speculation required as a postulate. Idealism, rejecting this
postulate, declared that Matter was simply the projection of Mind, and
that our Body was the objectivation of our Soul. Physiology began to
disclose that all the mental processes were (mathematically speaking)
_functions_ of physical processes, i. e. varying with the variations
of bodily states; and this was declared enough to banish forever the
conception of a Soul, except as a term simply expressing certain
functions.
11. Idealism and Materialism are equally destructive of Dualism. The
defects of particular idealist and materialist theories we will
not here touch upon; they mainly result from defects of Method. Not
sufficiently recognizing the primary fact testified by Consciousness,
namely, that Experience expresses both physical and mental aspects,
and that a Not-Self is everywhere indissolubly interwoven with Self,
an objective factor with a subjective factor, the idealist reduces
Existence to a mere panorama of mental states, and the Body to a
group in this panorama. He is thus incapable of giving a satisfactory
explanation of all the objective phenomena which do not follow in the
same order as his feelings, which manifest a succession unlike his
expectation, and which he cannot class under the order of his mental
states hitherto experienced. He conceives that it is the Mind which
_prescribes_ the order in Things; whereas experience assures us that
the order is _described_, not prescribed by us: described in terms of
Feeling, but determined by the laws of Things, i. e. the genesis of
subjective phenomena is determined by the action of the Cosmos on our
Sensibility, and the reaction of our Sensibility. He overlooks the
evidence that the mental forms or laws of thought which determine the
character of particular experiences, were themselves evolved through a
continual action and reaction of the Cosmos and the Soul, precisely as
the laws of organic action which determine the character of particular
functions were evolved through a continual adaptation of the organism
to the medium. These immanent laws are declared to be transcendental,
antecedent to all such action and reaction.
A similar exclusiveness vitiates the materialist doctrine. Overlooking
the primary fact that Feeling is indissolubly interwoven with processes
regarded as purely physical because they are considered solely in their
objective aspect, the materialist fails to recognize the operation of
psychological laws in the determination of physiological results;
he hopes to reduce Biology to a problem of Mechanics. But Vitality
and Sensibility are coefficients which must render the mechanical
problem insoluble, if only on the ground that mechanical principles
have reference to quantitative relations, whereas vital relations are
qualitative. His error is the obverse of the vitalist’s error. The
vitalist imagines that the speciality of organic phenomena proves the
existence of a cause which has no community with the forces operating
elsewhere; so, turning his back on all the evidence, he attempts to
explain organic phenomena without any aid from Physics and Chemistry.
The materialist, turning his back on all the evidence of quite special
conditions only found at work in living organisms, tries to explain
the problem solely by the aid of Physics and Chemistry. It is quite
certain that physiological and psychological problems are not to be
solved if we disregard the laws of Evolution through Epigenesis. The
mental structure is evolved, as the physical structure is evolved. It
is quite certain that no such evolution is visible in anorganisms,
nor will any one suppose it to be possible in machines. From the
biological point of view we must therefore reject both Idealism and
Materialism. We applaud the one when it says, “Don’t confuse mental
facts by the introduction of physical hypotheses”; and the other when
it says, “Don’t darken physical facts with metaphysical mists.” We say
to both, “By all means make clear to yourselves which aspect of the
phenomena you are dealing with, and express each in its own terms. But
in endeavoring to understand a phenomenon you must take into account
all its ascertainable conditions. Now these conditions are sometimes
only approachable from the objective side; at other times only from the
subjective side.”
12. While it is necessary to keep the investigation of a process on its
objective side, limited to objective conditions, and to express the
result in objective terms, we must remember that this is an artifice;
above all, we must remember that even within the objective limits our
analyses are only provisional, and must be finally rectified by a
restoration of all the elements we have provisionally set aside. Thus
rectified, the objective interpretation of vital and mental phenomena
has the incomparable advantage of simplifying research, keeping it
fixed on physical processes, instead of being perturbed by suggestions
of metaphysical processes. And as all physical investigation naturally
tends to reduce itself to a mechanical investigation, because Mechanics
is the science of motion, and all physical processes are motions,
we may be asked, Why should not the mechanical point of view be the
rational standing-point of the biologist? Our answer is, Because
Mechanics concerns itself with abstract relations, and treats of
products without reference to modes of production, i. e. with motions
without reference to all the conditions on which they depend. Every
physical change, if expressed in physical terms, is a change of
position, and is determined by some preceding change of position. It
is a movement having a certain velocity and direction, which velocity
and direction are determined by the velocity and direction of a force
(a pressure or a tension) compounded with the forces of resistance,
i. e. counter-pressures. Clearly, the nature of the forces in operation
must be taken into account; and it is this which the mechanical view
disregards, the biological regards. The mechanical view is fixed
on the ascertained adjustment of the parts, so that the working of
the organism may be explained as if it were a machine, a movement
here liberating a movement there. The biological view includes this
adjustment of parts, but takes in also the conditions of molecular
change in the parts on which the adjustment dynamically depends.
Mechanical actions may be expressed as the enlargement or diminution
of the angle of two levers; but chemical actions are not thus
expressible; still less vital and mental actions.
13. The organism is on the physical side a mechanism, and so long
as the mechanical interpretation of organic phenomena is confined
to expressing the mechanical principles involved in the mechanical
relations, it is eminently to be applauded. But the organism is
something more than a mechanism, even on the physical side; or, since
this statement may be misunderstood, let me say, what no one will
dispute, that the organism is a mechanism of a very special kind, in
many cardinal points unlike all machines. This difference of kind
brings with it a difference of causal conditions. In so far as the
actions of this mechanism are those of a dependent sequence of material
positions, they are actions expressible in mechanical terms; but in
so far as these actions are dependent on vital processes, they are
not expressible in mechanical terms. Vital facts, especially facts
of sensibility, have factors neither discernible in machines nor
expressible in mechanical terms. We cannot ignore them, although for
analytical purposes we may provisionally set them aside.
* * * * *
In the course of the development of the mechanical theory, the history
of which has just been briefly sketched, biological problems have more
and more come under its influence. There has always been a fierce
resistance to the attempt to explain vital and sentient phenomena
on mechanical, or even physical principles, but still the question
has incessantly recurred, How far is the organism mechanically
interpretable? And while the progress of Biology has shown more and
more the machine-like adjustment of the several parts of which the
organism is composed, it has also shown more and more the intervention
of conditions not mechanically interpretable. We shall have to
consider the question, therefore, under two forms. First, whether
animals are machines, and if not, by what characters do we distinguish
them from machines? Secondly, in what sense can we correctly speak of
Feeling as an agent in organic processes?
CHAPTER II.
THE VITAL MECHANISM.
14. No answer can be successfully attempted in reply to the first
of the questions which closed the last chapter until we have given
precision to certain terms of incessant recurrence. I have often
to remark on the peculiar misfortune of Psychology, that all its
principal terms are employed by different writers, and are understood
by different readers, in widely different senses: they denote and
connote meanings of various significance. All physicists mean the same
thing when they speak of weight, mass, momentum, electricity, heat,
etc. All chemists mean the same thing when they speak of affinity,
decomposition, oxygen, carbonic acid, etc. All physiologists mean the
same thing when they speak of muscle, nerve, nutrition, secretion, etc.
But scarcely any two psychologists mean precisely the same thing when
they speak of sensation, feeling, thought, volition, consciousness,
etc.; and the differences of denotation and connotation in their uses
of such terms lead to endless misunderstanding. As Rousseau says: “Les
définitions pourraient être bonnes si l’on n’employait pas les mots
pour les faire.” But since we must employ words as our signs, our
utmost care should be given to clearly marking what it is the signs
signify.
15. The question we have now before us, whether animal actions are
interpretable on purely mechanical principles? can only be answered
after a preliminary settlement of the terms. The first of these
terms to be settled is that of mechanism, when applied to the vital
organism. If the organism is a mechanism, its actions must of course be
interpretable on mechanical principles. But this general truth requires
a special interpretation, if on inquiry we find that the organism is a
particular kind of mechanism, one which is _not_ to be classed under
the same head as inorganic machines. And this we do find. In Problem
I. § 22, will be found a statement of the radical difference between
organic and inorganic mechanisms, due to the differences in their
structures. But the differences there noted do not affect the operation
of abstract mechanical principles, which are of course manifested
_wherever_ there is a dependent sequence of material changes; and which
are the same abstract principles in the mechanism of the heavens,
the mechanism of a paper-mill, or the mechanism of an animal body.
In other words, the principles are abstract, and are abstracted from
all concrete cases by letting drop what is special to each case,
retaining only what is common to all. This procedure is indispensable
to the ideal constructions of Science. But we cannot rightly interpret
any concrete case by abstract principles alone; we must restore the
special characters which the abstraction has eliminated. The most lucid
explanation of the mechanism of the heavens will leave us quite in
the dark respecting the action of a paper-mill, until we have studied
the mill at work, ascertained its structure and mode of operation,
and therein detected what is common both to its mechanism and to the
mechanism of the heavens. Thus equipped, we approach the study of
the animal mechanism, but find ourselves wholly in the dark until we
have also ascertained its structure and mode of operation; then we
may recognize in it the principles of dependent sequence which had
been abstracted from the paper-mill and the heavens. To neglect this
concrete study, and to argue from Machinery to Life in disregard
of special conditions, is not more rational than to assume that the
movement of a piston is prompted by volition.
16. The recognition of special differences is no denial of fundamental
identities. We do not deny the presence of phenomena in organisms
which belong to physical and chemical agencies, but we assert that
organisms have other phenomena besides these, dependent on conditions
not present in physical and chemical phenomena. The same material
elements and forces may be recognized in a moving inorganic body,
and a moving organic body; but in the latter there is a speciality
of combination with a speciality of result. Just as the same words
and laws of grammatical construction may be recognized in prose and
poetry; yet poetry is not prose, but has special rules of its own,
and special effects. In an organism, as in a machine, the adjustment
of the parts is a condition of the mechanical action; the one enables
us to explain the other. But the parts adjusted, and the consequences
of the adjustment, are unlike in the two cases. This unlikeness is
pervading and profound. One cardinal difference is that the combination
of the parts is in the machine a fixed, in the organism a fluctuating
adjustment; and this fluctuation is due to certain vital processes
subjectively known as _sensitive guidance_. Hence machines have fixed
and calculated mechanisms; whereas organisms are variable and to a
great extent incalculable mechanisms.
17. I conceive, therefore, that a theory which reduces vital activities
to purely physical processes is self-condemned. Not that we are
to admit the agency of any extra-organic principle, such as the
hypothesis of Vitalism assumes (Prob. I. § 14); but only the agency
of an intra-organic principle, or the abstract symbol of _all_ the
co-operant conditions--the special combination of forces which result
in organization. This assures us that an organism is a peculiar kind
of mechanism, the processes in which are peculiar to it; and among
those processes there is one which results in what we call Sensibility.
This Sensibility is a factor which raises the phenomena into another
order. To overlook its presence is fatal to any explanation of the
organic mechanism. Yet it is overlooked by those who tell us that
when an impression on a nerve is conveyed to the brain, and is thence
reflected on the limbs--as when the retina of a wolf is stimulated
by the image of a sheep, and the spring of the wolf upon the sheep
follows as a “purely mechanical consequence--the whole process has
from first to last been physical.” Unless the term _physical_ is
here used to designate the _objective sequence_, as contemplated by
an onlooker, who likens the process to the sequence observable in a
machine, I should say that from first to last the process has been
_not_ physical, but _vital_, involving among its essential conditions
the peculiarly vital factor named Sensibility. The process taking place
in the wolf’s organism is one which involves conditions never found in
purely physical processes. We may indeed analytically disregard these.
We may view the process in its purely physical relations, or in its
purely chemical relations, or in its purely mathematical (mechanical)
relations. But this is the artifice of the analytical method. In
reality the process is no one of these, for it is all of these; it is a
process in a living organism, and depends on conditions only found in
living organisms--nay, in this particular case the process depends on
conditions only found in organisms like that of the wolf; for the image
of the sheep will stimulate the brain of a goat, horse, or elephant
without producing any such movement in the organism.
18. The importance of this point must excuse my reiteration of
it. We must make clear to ourselves that the organism is in its
objective aspect a physiological mechanism, in its subjective aspect
a psychological mechanism: in both aspects it is to be radically
demarcated from all inorganic mechanisms. In it the combination
and co-ordination of movements involve conditions never present
in machines; among these conditions, there are combinations and
co-ordinations of Sensibility, which, although material processes
on the objective side, are processes believed to be only present in
organisms. We have the strongest reasons for concluding that every
feeling, every change in Sensibility, has its correlative material
process in the organism--is, in short, only the subjective aspect
of the objective organic change. What in Physiology is called
Co-ordination and has reference to movements, in Psychology may be
called Logic, having reference to feelings. But be this latter point
accepted or rejected, the one point which admits of no dispute is
that an organism is radically distinguishable from every inorganic
mechanism in that _it acquires through the very exercise of its primary
constitution, a new constitution with new powers_. Its adjustment is a
changing and developing mechanism. That is to say, a machine, however
complex its structure, is constructed once for all, and this primary
constitution is final, the adjustment of parts remaining unaltered;
and although by exercise the machine may come to work more easily,
with less friction, it never comes to work differently, to _readjust_
its parts, and develop new capabilities. It has no _historical_ factor
manifest in its functions. It has no experience. It reacts at last
as at first. How different the organism! This has not only variable
adjustments due to internal fluctuations, it has experience which
develops new parts, and new adjustments of old parts. Every organism
has its _primary_ constitution in the adjustment of parts peculiar to
the species; it has also its _secondary_ or modified constitution,
in the adjustment which has been more or less altered by individual
experiences; it has, thirdly, its _temporary_ constitution in the
variable adjustment due to the varying state of tension which results
from varying stimulation.
19. A word on each. There is a structural disposition of the parts
which is common to large groups of organisms, so that a corresponding
similarity is observable in the reactions of these organisms. Thus all
quadrupeds use their limbs for locomotion in very similar ways; birds
use their wings for flight in similar ways. All vertebrates swallow
their food, defend themselves, shrink when hurt, etc., in ways that are
very similar. In so far as their organizations are alike, their actions
and reactions are alike. In so far as their organizations differ, their
actions and reactions differ. The goose and the vulture are alike in
the main lines of structure; still more alike are duck and hen; yet,
owing to certain unlike characters of structure, they manifest some
marked differences in action and reaction: the goose will starve in
the presence of food which the vulture gluttonously devours, and the
vulture will refuse the vegetable food which the goose devours; the
duck plunges into the water, the hen not only refuses to enter it,
but is greatly agitated when she sees the ducklings she has hatched
plunging into it. That peculiar instincts, habits, and feelings are
rigorously determined by peculiarities in the organism, no one doubts,
when animals are in question. If this is less obvious in the case of
men, the reason is that there the influence of other factors somewhat
masks the operation of the primary constitution--these factors are the
modified and the temporary constitutions. Yet even in man it is true to
say that his feelings and actions are the result of his organization,
native and acquired.
20. No two men are organized in all respects alike. There are
individual variations in structure, both native and acquired. These may
be too slight to be appreciable by any other test than the difference
of reaction under similar external stimuli; but the variations in the
sensibility to music, color, temperature, sexual influence, moral
influence, etc., betray corresponding differences in the organisms.
Any one variation in structure, seemingly trivial, may be the origin
of well-marked diversity in physical and moral characters. Compare
the bull with the ox, or the predatory aggressive eagle with the
cowardly vulture. Nor are the temporary modifications to be overlooked.
Antoine Cros mentions the case of a patient, a young girl, suffering
from congested liver and spleen, which of course altered the state
of her blood, and thus for a time modified her constitution. Her
moral character was greatly altered by it. She ceased to feel any
affection for father or mother; would play with her doll, but could
not be brought to show any delight in it; could not be drawn out of
her apathetic sadness. Things which previously had made her shriek
with laughter, now left her uninterested. Her temper changed, became
capricious and violent.[207] Congestion of the lungs, if unaccompanied
by congestion of the liver, never produces such effects, because not
thus altering the blood. The effects of liver congestion are familiar.
Cros cites the case of a magistrate whose liver was enlarged, and whose
skin showed a markedly bilious aspect, and in whom all affection seemed
to be dead: he did not exhibit any perversion or violence, only want
of emotive reaction. If he went to the theatre he could not feel the
slightest pleasure in it. The thoughts of his home, his absent wife
and children, were, he declared, as unaffecting to him as a problem in
Euclid.
21. Owing to the recognized dependence of peculiar instincts and modes
of reaction on peculiarities of structure, comparative anatomists
are quite confident, when they find a portion of a skull with two
occipital condyles, that the animal to which this skull belonged had
red blood-corpuscles without nuclei, and (if a female) suckled its
young. If in that fragment of skull there remain a single tooth, it
will prove that the animal was carnivorous or herbivorous, and had, or
had not, retractile claws. From such data a general conclusion may be
formed as to the instincts and habits of the animal. The data disclose
much of the primary constitution, that is to say, the mechanism
which the animal brought with it into the world, ready prepared to
react in definite ways on being stimulated. The connate mechanism
has correlative tendencies of reaction. Some of these tendencies are
inevitably called into play by external conditions, and they continue
unaltered amid great varieties of circumstances, provided none of these
variations directly deprive them of their appropriate stimulation.
Such tendencies of the connate mechanism are styled _automatic_ (an
unfortunate metaphor, which has led to the theory of Automatism), and
include, besides the visceral reactions, the more complex reactions
of winking, breathing, swallowing, coughing, flying, walking, etc. It
is true that we learn to walk, and learn to wink, whereas the other
actions require no tentative efforts directed by experience; but the
mechanism of all these actions is already laid down in the primary
constitution, and is inevitably called into play.