THE NEW SCIENCE OF EXPERIMENTAL PSYCHOLOGY
BRAIN AND MIND
A little over a hundred years ago a reform movement was afoot in
the world in the interests of the insane. As was fitting, the
movement showed itself first in America, where these unfortunates
were humanely cared for at a time when their treatment elsewhere
was worse than brutal; but England and France quickly fell into
line. The leader on this side of the water was the famous
Philadelphian, Dr. Benjamin Rush, "the Sydenham of America"; in
England, Dr. William Tuke inaugurated the movement; and in
France, Dr. Philippe Pinel, single-handed, led the way. Moved by
a common spirit, though acting quite independently, these men
raised a revolt against the traditional custom which, spurning
the insane as demon-haunted outcasts, had condemned these
unfortunates to dungeons, chains, and the lash. Hitherto few
people had thought it other than the natural course of events
that the "maniac" should be thrust into a dungeon, and perhaps
chained to the wall with the aid of an iron band riveted
permanently about his neck or waist. Many an unfortunate, thus
manacled, was held to the narrow limits of his chain for years
together in a cell to which full daylight never penetrated;
sometimes--iron being expensive--the chain was so short that the
wretched victim could not rise to the upright posture or even
shift his position upon his squalid pallet of straw.
In America, indeed, there being no Middle Age precedents to
crystallize into established customs, the treatment accorded the
insane had seldom or never sunk to this level. Partly for this
reason, perhaps, the work of Dr. Rush at the Philadelphia
Hospital, in 1784, by means of which the insane came to be
humanely treated, even to the extent of banishing the lash, has
been but little noted, while the work of the European leaders,
though belonging to later decades, has been made famous. And
perhaps this is not as unjust as it seems, for the step which
Rush took, from relatively bad to good, was a far easier one to
take than the leap from atrocities to good treatment which the
European reformers were obliged to compass. In Paris, for
example, Pinel was obliged to ask permission of the authorities
even to make the attempt at liberating the insane from their
chains, and, notwithstanding his recognized position as a leader
of science, he gained but grudging assent, and was regarded as
being himself little better than a lunatic for making so
manifestly unwise and hopeless an attempt. Once the attempt had
been made, however, and carried to a successful issue, the
amelioration wrought in the condition of the insane was so patent
that the fame of Pinel's work at the Bicetre and the Salpetriere
went abroad apace. It required, indeed, many years to complete it
in Paris, and a lifetime of effort on the part of Pinel's pupil
Esquirol and others to extend the reform to the provinces; but
the epochal turning-point had been reached with Pinel's labors of
the closing years of the eighteenth century.
The significance of this wise and humane reform, in the present
connection, is the fact that these studies of the insane gave
emphasis to the novel idea, which by-and-by became accepted as
beyond question, that "demoniacal possession" is in reality no
more than the outward expression of a diseased condition of the
brain. This realization made it clear, as never before, how
intimately the mind and the body are linked one to the other.
And so it chanced that, in striking the shackles from the insane,
Pinel and his confreres struck a blow also, unwittingly, at
time-honored philosophical traditions. The liberation of the
insane from their dungeons was an augury of the liberation of
psychology from the musty recesses of metaphysics. Hitherto
psychology, in so far as it existed at all, was but the
subjective study of individual minds; in future it must become
objective as well, taking into account also the relations which
the mind bears to the body, and in particular to the brain and
nervous system.
The necessity for this collocation was advocated quite as
earnestly, and even more directly, by another worker of this
period, whose studies were allied to those of alienists, and who,
even more actively than they, focalized his attention upon the
brain and its functions. This earliest of specialists in brain
studies was a German by birth but Parisian by adoption, Dr. Franz
Joseph Gall, originator of the since-notorious system of
phrenology. The merited disrepute into which this system has
fallen through the exposition of peripatetic charlatans should
not make us forget that Dr. Gall himself was apparently a highly
educated physician, a careful student of the brain and mind
according to the best light of his time, and, withal, an earnest
and honest believer in the validity of the system he had
originated. The system itself, taken as a whole, was hopelessly
faulty, yet it was not without its latent germ of truth, as later
studies were to show. How firmly its author himself believed in
it is evidenced by the paper which he contributed to the French
Academy of Sciences in 1808. The paper itself was referred to a
committee of which Pinel and Cuvier were members. The verdict of
this committee was adverse, and justly so; yet the system
condemned had at least one merit which its detractors failed to
realize. It popularized the conception that the brain is the
organ of mind. Moreover, by its insistence it rallied about it a
band of scientific supporters, chief of whom was Dr. Kaspar
Spurzlieim, a man of no mean abilities, who became the
propagandist of phrenology in England and in America. Of course
such advocacy and popularity stimulated opposition as well, and
out of the disputations thus arising there grew presently a
general interest in the brain as the organ of mind, quite aside
from any preconceptions whatever as to the doctrines of Gall and
Spurzheim.
Prominent among the unprejudiced class of workers who now
appeared was the brilliant young Frenchman Louis Antoine
Desmoulins, who studied first under the tutorage of the famous
Magendie, and published jointly with him a classical work on the
nervous system of vertebrates in 1825. Desmoulins made at least
one discovery of epochal importance. He observed that the brains
of persons dying in old age were lighter than the average and
gave visible evidence of atrophy, and he reasoned that such decay
is a normal accompaniment of senility. No one nowadays would
question the accuracy of this observation, but the scientific
world was not quite ready for it in 1825; for when Desmoulins
announced his discovery to the French Academy, that august and
somewhat patriarchal body was moved to quite unscientific wrath,
and forbade the young iconoclast the privilege of further
hearings. From which it is evident that the partially liberated
spirit of the new psychology had by no means freed itself
altogether, at the close of the first quarter of the nineteenth
century, from the metaphysical cobwebs of its long incarceration.
FUNCTIONS OF THE NERVES
While studies of the brain were thus being inaugurated, the
nervous system, which is the channel of communication between the
brain and the outside world, was being interrogated with even
more tangible results. The inaugural discovery was made in 1811
by Dr. (afterwards Sir Charles) Bell,[1] the famous English
surgeon and experimental physiologist. It consisted of the
observation that the anterior roots of the spinal nerves are
given over to the function of conveying motor impulses from the
brain outward, whereas the posterior roots convey solely sensory
impulses to the brain from without. Hitherto it had been supposed
that all nerves have a similar function, and the peculiar
distribution of the spinal nerves had been an unsolved puzzle.
Bell's discovery was epochal; but its full significance was not
appreciated for a decade, nor, indeed, was its validity at first
admitted. In Paris, in particular, then the court of final
appeal in all matters scientific, the alleged discovery was
looked at askance, or quite ignored. But in 1823 the subject was
taken up by the recognized leader of French physiology--Francois
Magendie--in the course of his comprehensive experimental studies
of the nervous system, and Bell's conclusions were subjected to
the most rigid experimental tests and found altogether valid.
Bell himself, meanwhile, had turned his attention to the cranial
nerves, and had proved that these also are divisible into two
sets--sensory and motor. Sometimes, indeed, the two sets of
filaments are combined into one nerve cord, but if traced to
their origin these are found to arise from different brain
centres. Thus it was clear that a hitherto unrecognized duality
of function pertains to the entire extra-cranial nervous system.
Any impulse sent from the periphery to the brain must be conveyed
along a perfectly definite channel; the response from the brain,
sent out to the peripheral muscles, must traverse an equally
definite and altogether different course. If either channel is
interrupted--as by the section of its particular nerve tract--the
corresponding message is denied transmission as effectually as an
electric current is stopped by the section of the transmitting
wire.
Experimenters everywhere soon confirmed the observations of Bell
and Magendie, and, as always happens after a great discovery, a
fresh impulse was given to investigations in allied fields.
Nevertheless, a full decade elapsed before another discovery of
comparable importance was made. Then Marshall Hall, the most
famous of English physicians of his day, made his classical
observations on the phenomena that henceforth were to be known as
reflex action. In 1832, while experimenting one day with a
decapitated newt, he observed that the headless creature's limbs
would contract in direct response to certain stimuli. Such a
response could no longer be secured if the spinal nerves
supplying a part were severed. Hence it was clear that responsive
centres exist in the spinal cord capable of receiving a sensory
message and of transmitting a motor impulse in reply--a function
hitherto supposed to be reserved for the brain. Further studies
went to show that such phenomena of reflex action on the part of
centres lying outside the range of consciousness, both in the
spinal cord and in the brain itself, are extremely common; that,
in short, they enter constantly into the activities of every
living organism and have a most important share in the sum total
of vital movements. Hence, Hall's discovery must always stand as
one of the great mile-stones of the advance of neurological
science.
Hall gave an admirably clear and interesting account of his
experiments and conclusions in a paper before the Royal Society,
"On the Reflex Functions of the Medulla Oblongata and the Medulla
Spinalis," from which, as published in the Transactions of the
society for 1833, we may quote at some length:
"In the entire animal, sensation and voluntary motion, functions
of the cerebrum, combine with the functions of the medulla
oblongata and medulla spinalis, and may therefore render it
difficult or impossible to determine those which are peculiar to
each; if, in an animal deprived of the brain, the spinal marrow
or the nerves supplying the muscles be stimulated, those muscles,
whether voluntary or respiratory, are equally thrown into
contraction, and, it may be added, equally in the complete and in
the mutilated animal; and, in the case of the nerves, equally in
limbs connected with and detached from the spinal marrow.
"The operation of all these various causes may be designated
centric, as taking place AT, or at least in a direction FROM,
central parts of the nervous system. But there is another
function the phenomena of which are of a totally different order
and obey totally different laws, being excited by causes in a
situation which is EXCENTRIC in the nervous system--that is,
distant from the nervous centres. This mode of action has not, I
think, been hitherto distinctly understood by physiologists.
"Many of the phenomena of this principle of action, as they occur
in the limbs, have certainly been observed. But, in the first
place, this function is by no means confined to the limbs; for,
while it imparts to each muscle its appropriate tone, and to each
system of muscles its appropriate equilibrium or balance, it
performs the still more important office of presiding over the
orifices and terminations of each of the internal canals in the
animal economy, giving them their due form and action; and, in
the second place, in the instances in which the phenomena of this
function have been noticed, they have been confounded, as I have
stated, with those of sensation and volition; or, if they have
been distinguished from these, they have been too indefinitely
denominated instinctive, or automatic. I have been compelled,
therefore, to adopt some new designation for them, and I shall
now give the reasons for my choice of that which is given in the
title of this paper--'Reflex Functions.'
"This property is characterized by being EXCITED in its action
and REFLEX in its course: in every instance in which it is
exerted an impression made upon the extremities of certain nerves
is conveyed to the medulla oblongata or the medulla spinalis, and
is reflected along the nerves to parts adjacent to, or remote
from, that which has received the impression.
"It is by this reflex character that the function to which I have
alluded is to be distinguished from every other. There are, in
the animal economy, four modes of muscular action, of muscular
contraction. The first is that designated VOLUNTARY: volition,
originated in the cerebrum and spontaneous in its acts, extends
its influence along the spinal marrow and the motor nerves in a
DIRECT LINE to the voluntary muscles. The SECOND is that of
RESPIRATION: like volition, the motive influence in respiration
passes in a DIRECT LINE from one point of the nervous system to
certain muscles; but as voluntary motion seems to originate in
the cerebrum, so the respiratory motions originate in the medulla
oblongata: like the voluntary motions, the motions of
respirations are spontaneous; they continue, at least, after the
eighth pair of nerves have been divided. The THIRD kind of
muscular action in the animal economy is that termed involuntary:
it depends upon the principle of irritability and requires the
IMMEDIATE application of a stimulus to the nervo-muscular fibre
itself. These three kinds of muscular motion are well known to
physiologists; and I believe they are all which have been
hitherto pointed out. There is, however, a FOURTH, which
subsists, in part, after the voluntary and respiratory motions
have ceased, by the removal of the cerebrum and medulla
oblongata, and which is attached to the medulla spinalis, ceasing
itself when this is removed, and leaving the irritability
undiminished. In this kind of muscular motion the motive
influence does not originate in any central part of the nervous
system, but from a distance from that centre; it is neither
spontaneous in its action nor direct in its course; it is, on the
contrary, EXCITED by the application of appropriate stimuli,
which are not, however, applied immediately to the muscular or
nervo-muscular fibre, but to certain membraneous parts, whence
the impression is carried through the medulla, REFLECTED and
reconducted to the part impressed, or conducted to a part remote
from it in which muscular contraction is effected.
"The first three modes of muscular action are known only by
actual movements of muscular contractions. But the reflex
function exists as a continuous muscular action, as a power
presiding over organs not actually in a state of motion,
preserving in some, as the glottis, an open, in others, as the
sphincters, a closed form, and in the limbs a due degree of
equilibrium or balanced muscular action--a function not, I think,
hitherto recognized by physiologists.
The three kinds of muscular motion hitherto known may be
distinguished in another way. The muscles of voluntary motion
and of respiration may be excited by stimulating the nerves which
supply them, in any part of their course, whether at their source
as a part of the medulla oblongata or the medulla spinalis or
exterior to the spinal canal: the muscles of involuntary motion
are chiefly excited by the actual contact of stimuli. In the
case of the reflex function alone the muscles are excited by a
stimulus acting mediately and indirectly in a curved and reflex
course, along superficial subcutaneous or submucous nerves
proceeding from the medulla. The first three of these causes of
muscular motion may act on detached limbs or muscles. The last
requires the connection with the medulla to be preserved entire.
"All the kinds of muscular motion may be unduly excited, but the
reflex function is peculiar in being excitable in two modes of
action, not previously subsisting in the animal economy, as in
the case of sneezing, coughing, vomiting, etc. The reflex
function also admits of being permanently diminished or augmented
and of taking on some other morbid forms, of which I shall treat
hereafter.
"Before I proceed to the details of the experiments upon which
this disposition rests, it may be well to point out several
instances in illustration of the various sources of and the modes
of muscular action which have been enumerated. None can be more
familiar than the act of swallowing. Yet how complicated is the
act! The apprehension of the food by the teeth and tongue, etc.,
is voluntary, and cannot, therefore, take place in an animal from
which the cerebrum is removed. The transition of food over the
glottis and along the middle and lower part of the pharynx
depends upon the reflex action: it can take place in animals from
which the cerebrum has been removed or the ninth pair of nerves
divided; but it requires the connection with the medulla
oblongata to be preserved entirely; and the actual contact of
some substance which may act as a stimulus: it is attended by
the accurate closure of the glottis and by the contraction of the
pharynx. The completion of the act of deglutition is dependent
upon the stimulus immediately impressed upon the muscular fibre
of the oesophagus, and is the result of excited irritability.
"However plain these observations may have made the fact that
there is a function of the nervous muscular system distinct from
sensation, from the voluntary and respiratory motions, and from
irritability, it is right, in every such inquiry as the present,
that the statements and reasonings should be made with the
experiment, as it were, actually before us. It has already been
remarked that the voluntary and respiratory motions are
spontaneous, not necessarily requiring the agency of a stimulus.
If, then, an animal can be placed in such circumstances that such
motions will certainly not take place, the power of moving
remaining, it may be concluded that volition and the motive
influence of respiration are annihilated. Now this is effected by
removing the cerebrum and the medulla oblongata. These facts are
fully proved by the experiments of Legallois and M. Flourens, and
by several which I proceed to detail, for the sake of the
opportunity afforded by doing so of stating the arguments most
clearly.
"I divided the spinal marrow of a very lively snake between the
second and third vertebrae. The movements of the animal were
immediately before extremely vigorous and unintermitted. From the
moment of the division of the spinal marrow it lay perfectly
tranquil and motionless, with the exception of occasional
gaspings and slight movements of the head. It became quite
evident that this state of quiescence would continue indefinitely
were the animal secured from all external impressions.
"Being now stimulated, the body began to move with great
activity, and continued to do so for a considerable time, each
change of position or situation bringing some fresh part of the
surface of the animal into contact with the table or other
objects and renewing the application of stimulants.
"At length the animal became again quiescent; and being carefully
protected from all external impressions it moved no more, but
died in the precise position and form which it had last assumed.
"It requires a little manoeuvre to perform this experiment
successfully: the motions of the animal must be watched and
slowly and cautiously arrested by opposing some soft substance,
as a glove or cotton wool; they are by this means gradually
lulled into quiescence. The slightest touch with a hard
substance, the slightest stimulus, will, on the other hand, renew
the movements on the animal in an active form. But that this
phenomenon does not depend upon sensation is further fully proved
by the facts that the position last assumed, and the stimuli, may
be such as would be attended by extreme or continued pain, if the
sensibility were undestroyed: in one case the animal remained
partially suspended over the acute edge of the table; in others
the infliction of punctures and the application of a lighted
taper did not prevent the animal, still possessed of active
powers of motion, from passing into a state of complete and
permanent quiescence."
In summing up this long paper Hall concludes with this sentence:
"The reflex function appears in a word to be the COMPLEMENT of
the functions of the nervous system hitherto known."[2]
All these considerations as to nerve currents and nerve tracts
becoming stock knowledge of science, it was natural that interest
should become stimulated as to the exact character of these nerve
tracts in themselves, and all the more natural in that the
perfected microscope was just now claiming all fields for its
own. A troop of observers soon entered upon the study of the
nerves, and the leader here, as in so many other lines of
microscopical research, was no other than Theodor Schwann.
Through his efforts, and with the invaluable aid of such other
workers as Remak, Purkinje, Henle, Muller, and the rest, all the
mystery as to the general characteristics of nerve tracts was
cleared away. It came to be known that in its essentials a nerve
tract is a tenuous fibre or thread of protoplasm stretching
between two terminal points in the organism, one of such termini
being usually a cell of the brain or spinal cord, the other a
distribution-point at or near the periphery--for example, in a
muscle or in the skin. Such a fibril may have about it a
protective covering, which is known as the sheath of Schwann; but
the fibril itself is the essential nerve tract; and in many
cases, as Remak presently discovered, the sheath is dispensed
with, particularly in case of the nerves of the so-called
sympathetic system.
This sympathetic system of ganglia and nerves, by-the-bye, had
long been a puzzle to the physiologists. Its ganglia, the
seeming centre of the system, usually minute in size and never
very large, are found everywhere through the organism, but in
particular are gathered into a long double chain which lies
within the body cavity, outside the spinal column, and represents
the sole nervous system of the non-vertebrated organisms. Fibrils
from these ganglia were seen to join the cranial and spinal nerve
fibrils and to accompany them everywhere, but what special
function they subserved was long a mere matter of conjecture and
led to many absurd speculations. Fact was not substituted for
conjecture until about the year 1851, when the great Frenchman
Claude Bernard conclusively proved that at least one chief
function of the sympathetic fibrils is to cause contraction of
the walls of the arterioles of the system, thus regulating the
blood-supply of any given part. Ten years earlier Henle had
demonstrated the existence of annular bands of muscle fibres in
the arterioles, hitherto a much-mooted question, and several
tentative explanations of the action of these fibres had been
made, particularly by the brothers Weber, by Stilling, who, as
early as 1840, had ventured to speak of "vaso-motor" nerves, and
by Schiff, who was hard upon the same track at the time of
Bernard's discovery. But a clear light was not thrown on the
subject until Bernard's experiments were made in 1851. The
experiments were soon after confirmed and extended by
Brown-Sequard, Waller, Budge, and numerous others, and henceforth
physiologists felt that they understood how the blood-supply of
any given part is regulated by the nervous system.
In reality, however, they had learned only half the story, as
Bernard himself proved only a few years later by opening up a new
and quite unsuspected chapter. While experimenting in 1858 he
discovered that there are certain nerves supplying the heart
which, if stimulated, cause that organ to relax and cease
beating. As the heart is essentially nothing more than an
aggregation of muscles, this phenomenon was utterly puzzling and
without precedent in the experience of physiologists. An impulse
travelling along a motor nerve had been supposed to be able to
cause a muscular contraction and to do nothing else; yet here
such an impulse had exactly the opposite effect. The only tenable
explanation seemed to be that this particular impulse must arrest
or inhibit the action of the impulses that ordinarily cause the
heart muscles to contract. But the idea of such inhibition of one
impulse by another was utterly novel and at first difficult to
comprehend. Gradually, however, the idea took its place in the
current knowledge of nerve physiology, and in time it came to be
understood that what happens in the case of the heart
nerve-supply is only a particular case under a very general,
indeed universal, form of nervous action. Growing out of
Bernard's initial discovery came the final understanding that the
entire nervous system is a mechanism of centres subordinate and
centres superior, the action of the one of which may be
counteracted and annulled in effect by the action of the other.
This applies not merely to such physical processes as heart-beats
and arterial contraction and relaxing, but to the most intricate
functionings which have their counterpart in psychical processes
as well. Thus the observation of the inhibition of the heart's
action by a nervous impulse furnished the point of departure for
studies that led to a better understanding of the modus operandi
of the mind's activities than had ever previously been attained
by the most subtle of psychologists.
PSYCHO-PHYSICS
The work of the nerve physiologists had thus an important bearing
on questions of the mind. But there was another company of
workers of this period who made an even more direct assault upon
the "citadel of thought." A remarkable school of workers had been
developed in Germany, the leaders being men who, having more or
less of innate metaphysical bias as a national birthright, had
also the instincts of the empirical scientist, and whose
educational equipment included a profound knowledge not alone of
physiology and psychology, but of physics and mathematics as
well. These men undertook the novel task of interrogating the
relations of body and mind from the standpoint of physics. They
sought to apply the vernier and the balance, as far as might be,
to the intangible processes of mind.
The movement had its precursory stages in the early part of the
century, notably in the mathematical psychology of Herbart, but
its first definite output to attract general attention came from
the master-hand of Hermann Helmholtz in 1851. It consisted of the
accurate measurement of the speed of transit of a nervous impulse
along a nerve tract. To make such measurement had been regarded
as impossible, it being supposed that the flight of the nervous
impulse was practically instantaneous. But Helmholtz readily
demonstrated the contrary, showing that the nerve cord is a
relatively sluggish message-bearer. According to his experiments,
first performed upon the frog, the nervous "current" travels less
than one hundred feet per second. Other experiments performed
soon afterwards by Helmholtz himself, and by various followers,
chief among whom was Du Bois-Reymond, modified somewhat the exact
figures at first obtained, but did not change the general
bearings of the early results. Thus the nervous impulse was shown
to be something far different, as regards speed of transit, at
any rate, from the electric current to which it had been so often
likened. An electric current would flash halfway round the globe
while a nervous impulse could travel the length of the human
body--from a man's foot to his brain.
The tendency to bridge the gulf that hitherto had separated the
physical from the psychical world was further evidenced in the
following decade by Helmholtz's remarkable but highly technical
study of the sensations of sound and of color in connection with
their physical causes, in the course of which he revived the
doctrine of color vision which that other great physiologist and
physicist, Thomas Young, had advanced half a century before. The
same tendency was further evidenced by the appearance, in 1852,
of Dr. Hermann Lotze's famous Medizinische Psychologie, oder
Physiologie der Seele, with its challenge of the old myth of a
"vital force." But the most definite expression of the new
movement was signalized in 1860, when Gustav Fechner published
his classical work called Psychophysik. That title introduced a
new word into the vocabulary of science. Fechner explained it by
saying, "I mean by psychophysics an exact theory of the relation
between spirit and body, and, in a general way, between the
physical and the psychic worlds." The title became famous and the
brunt of many a controversy. So also did another phrase which
Fechner introduced in the course of his book--the phrase
"physiological psychology." In making that happy collocation of
words Fechner virtually christened a new science.
FECHNER EXPOUNDS WEBER'S LAW
The chief purport of this classical book of the German
psycho-physiologist was the elaboration and explication of
experiments based on a method introduced more than twenty years
earlier by his countryman E. H. Weber, but which hitherto had
failed to attract the attention it deserved. The method consisted
of the measurement and analysis of the definite relation existing
between external stimuli of varying degrees of intensity (various
sounds, for example) and the mental states they induce. Weber's
experiments grew out of the familiar observation that the nicety
of our discriminations of various sounds, weights, or visual
images depends upon the magnitude of each particular cause of a
sensation in its relation with other similar causes. Thus, for
example, we cannot see the stars in the daytime, though they
shine as brightly then as at night. Again, we seldom notice the
ticking of a clock in the daytime, though it may become almost
painfully audible in the silence of the night. Yet again, the
difference between an ounce weight and a two-ounce weight is
clearly enough appreciable when we lift the two, but one cannot
discriminate in the same way between a five-pound weight and a
weight of one ounce over five pounds.
This last example, and similar ones for the other senses, gave
Weber the clew to his novel experiments. Reflection upon
every-day experiences made it clear to him that whenever we
consider two visual sensations, or two auditory sensations, or
two sensations of weight, in comparison one with another, there
is always a limit to the keenness of our discrimination, and that
this degree of keenness varies, as in the case of the weights
just cited, with the magnitude of the exciting cause.
Weber determined to see whether these common experiences could be
brought within the pale of a general law. His method consisted of
making long series of experiments aimed at the determination, in
each case, of what came to be spoken of as the least observable
difference between the stimuli. Thus if one holds an ounce weight
in each hand, and has tiny weights added to one of them, grain by
grain, one does not at first perceive a difference; but
presently, on the addition of a certain grain, he does become
aware of the difference. Noting now how many grains have been
added to produce this effect, we have the weight which represents
the least appreciable difference when the standard is one ounce.
Now repeat the experiment, but let the weights be each of five
pounds. Clearly in this case we shall be obliged to add not
grains, but drachms, before a difference between the two heavy
weights is perceived. But whatever the exact amount added, that
amount represents the stimulus producing a just-perceivable
sensation of difference when the standard is five pounds. And so
on for indefinite series of weights of varying magnitudes. Now
came Weber's curious discovery. Not only did he find that in
repeated experiments with the same pair of weights the measure of
"just-{p}erceivable difference" remained approximately fixed, but
he found, further, that a remarkable fixed relation exists
between the stimuli of different magnitude. If, for example, he
had found it necessary, in the case of the ounce weights, to add
one-fiftieth of an ounce to the one before a difference was
detected, he found also, in the case of the five-pound weights,
that one-fiftieth of five pounds must be added before producing
the same result. And so of all other weights; the amount added
to produce the stimulus of "least-appreciable difference" always
bore the same mathematical relation to the magnitude of the
weight used, be that magnitude great or small.
Weber found that the same thing holds good for the stimuli of the
sensations of sight and of hearing, the differential stimulus
bearing always a fixed ratio to the total magnitude of the
stimuli. Here, then, was the law he had sought.
Weber's results were definite enough and striking enough, yet
they failed to attract any considerable measure of attention
until they were revived and extended by Fechner and brought
before the world in the famous work on psycho-physics. Then they
precipitated a veritable melee. Fechner had not alone verified
the earlier results (with certain limitations not essential to
the present consideration), but had invented new methods of
making similar tests, and had reduced the whole question to
mathematical treatment. He pronounced Weber's discovery the
fundamental law of psycho-physics. In honor of the discoverer, he
christened it Weber's Law. He clothed the law in words and in
mathematical formulae, and, so to say, launched it full tilt at
the heads of the psychological world. It made a fine commotion,
be assured, for it was the first widely heralded bulletin of the
new psychology in its march upon the strongholds of the
time-honored metaphysics. The accomplishments of the
microscopists and the nerve physiologists had been but
preliminary--mere border skirmishes of uncertain import. But here
was proof that the iconoclastic movement meant to invade the very
heart of the sacred territory of mind--a territory from which
tangible objective fact had been supposed to be forever barred.
PHYSIOLOGICAL PSYCHOLOGY
Hardly had the alarm been sounded, however, before a new movement
was made. While Fechner's book was fresh from the press, steps
were being taken to extend the methods of the physicist in yet
another way to the intimate processes of the mind. As Helmholtz
had shown the rate of nervous impulsion along the nerve tract to
be measurable, it was now sought to measure also the time
required for the central nervous mechanism to perform its work of
receiving a message and sending out a response. This was coming
down to the very threshold of mind. The attempt was first made by
Professor Donders in 1861, but definitive results were only
obtained after many years of experiment on the part of a host of
observers. The chief of these, and the man who has stood in the
forefront of the new movement and has been its recognized leader
throughout the remainder of the century, is Dr. Wilhelm Wundt, of
Leipzig.
The task was not easy, but, in the long run, it was accomplished.
Not alone was it shown that the nerve centre requires a
measurable time for its operations, but much was learned as to
conditions that modify this time. Thus it was found that
different persons vary in the rate of their central nervous
activity--which explained the "personal equation" that the
astronomer Bessel had noted a half-century before. It was found,
too, that the rate of activity varies also for the same person
under different conditions, becoming retarded, for example, under
influence of fatigue, or in case of certain diseases of the
brain. All details aside, the essential fact emerges, as an
experimental demonstration, that the intellectual
processes--sensation, apperception, volition--are linked
irrevocably with the activities of the central nervous tissues,
and that these activities, like all other physical processes,
have a time element. To that old school of psychologists, who
scarcely cared more for the human head than for the heels--being
interested only in the mind--such a linking of mind and body as
was thus demonstrated was naturally disquieting. But whatever the
inferences, there was no escaping the facts.
Of course this new movement has not been confined to Germany.
Indeed, it had long had exponents elsewhere. Thus in England, a
full century earlier, Dr. Hartley had championed the theory of
the close and indissoluble dependence of the mind upon the brain,
and formulated a famous vibration theory of association that
still merits careful consideration. Then, too, in France, at the
beginning of the century, there was Dr. Cabanis with his
tangible, if crudely phrased, doctrine that the brain digests
impressions and secretes thought as the stomach digests food and
the liver secretes bile. Moreover, Herbert Spencer's Principles
of Psychology, with its avowed co-ordination of mind and body and
its vitalizing theory of evolution, appeared in 1855, half a
decade before the work of Fechner. But these influences, though
of vast educational value, were theoretical rather than
demonstrative, and the fact remains that the experimental work
which first attempted to gauge mental operations by physical
principles was mainly done in Germany. Wundt's Physiological
Psychology, with its full preliminary descriptions of the anatomy
of the nervous system, gave tangible expression to the growth of
the new movement in 1874; and four years later, with the opening
of his laboratory of physiological psychology at the University
of Leipzig, the new psychology may be said to have gained a
permanent foothold and to have forced itself into official
recognition. From then on its conquest of the world was but a
matter of time.
It should be noted, however, that there is one other method of
strictly experimental examination of the mental field, latterly
much in vogue, which had a different origin. This is the
scientific investigation of the phenomena of hypnotism. This
subject was rescued from the hands of charlatans, rechristened,
and subjected to accurate investigation by Dr. James Braid, of
Manchester, as early as 1841. But his results, after attracting
momentary attention, fell from view, and, despite desultory
efforts, the subject was not again accorded a general hearing
from the scientific world until 1878, when Dr. Charcot took it up
at the Salpetriere, in Paris, followed soon afterwards by Dr.
Rudolf Heidenhain, of Breslau, and a host of other experimenters.
The value of the method in the study of mental states was soon
apparent. Most of Braid's experiments were repeated, and in the
main his results were confirmed. His explanation of hypnotism,
or artificial somnambulism, as a self-induced state, independent
of any occult or supersensible influence, soon gained general
credence. His belief that the initial stages are due to fatigue
of nervous centres, usually from excessive stimulation, has not
been supplanted, though supplemented by notions growing out of
the new knowledge as to subconscious mentality in general, and
the inhibitory influence of one centre over another in the
central nervous mechanism.
THE BRAIN AS THE ORGAN OF MIND
These studies of the psychologists and pathologists bring the
relations of mind and body into sharp relief. But even more
definite in this regard was the work of the brain physiologists.
Chief of these, during the middle period of the century, was the
man who is sometimes spoken of as the "father of brain
physiology," Marie Jean Pierre Flourens, of the Jardin des
Plantes of Paris, the pupil and worthy successor of Magendie.
His experiments in nerve physiology were begun in the first
quarter of the century, but his local experiments upon the brain
itself were not culminated until about 1842. At this time the old
dispute over phrenology had broken out afresh, and the studies of
Flourens were aimed, in part at least, at the strictly scientific
investigation of this troublesome topic.
In the course of these studies Flourens discovered that in the
medulla oblongata, the part of the brain which connects that
organ with the spinal cord, there is a centre of minute size
which cannot be injured in the least without causing the instant
death of the animal operated upon. It may be added that it is
this spot which is reached by the needle of the garroter in
Spanish executions, and that the same centre also is destroyed
when a criminal is "successfully" hanged, this time by the forced
intrusion of a process of the second cervical vertebra. Flourens
named this spot the "vital knot." Its extreme importance, as is
now understood, is due to the fact that it is the centre of
nerves that supply the heart; but this simple explanation,
annulling the conception of a specific "life centre," was not at
once apparent.
Other experiments of Flourens seemed to show that the cerebellum
is the seat of the centres that co-ordinate muscular activities,
and that the higher intellectual faculties are relegated to the
cerebrum. But beyond this, as regards localization, experiment
faltered. Negative results, as regards specific faculties, were
obtained from all localized irritations of the cerebrum, and
Flourens was forced to conclude that the cerebral lobe, while
being undoubtedly the seat of higher intellection, performs its
functions with its entire structure. This conclusion, which
incidentally gave a quietus to phrenology, was accepted
generally, and became the stock doctrine of cerebral physiology
for a generation.
It will be seen, however, that these studies of Flourens had a
double bearing. They denied localization of cerebral functions,
but they demonstrated the localization of certain nervous
processes in other portions of the brain. On the whole, then,
they spoke positively for the principle of localization of
function in the brain, for which a certain number of students
contended; while their evidence against cerebral localization was
only negative. There was here and there an observer who felt that
this negative testimony was not conclusive. In particular, the
German anatomist Meynert, who had studied the disposition of
nerve tracts in the cerebrum, was led to believe that the
anterior portions of the cerebrum must have motor functions in
preponderance; the posterior positions, sensory functions.
Somewhat similar conclusions were reached also by Dr.
Hughlings-Jackson, in England, from his studies of epilepsy. But
no positive evidence was forthcoming until 1861, when Dr. Paul
Broca brought before the Academy of Medicine in Paris a case of
brain lesion which he regarded as having most important bearings
on the question of cerebral localization.
The case was that of a patient at the Bicetre, who for twenty
years had been deprived of the power of speech, seemingly through
loss of memory of words. In 1861 this patient died, and an
autopsy revealed that a certain convolution of the left frontal
lobe of his cerebrum had been totally destroyed by disease, the
remainder of his brain being intact. Broca felt that this
observation pointed strongly to a localization of the memory of
words in a definite area of the brain. Moreover, it transpired
that the case was not without precedent. As long ago as 1825 Dr.
Boillard had been led, through pathological studies, to locate
definitely a centre for the articulation of words in the frontal
lobe, and here and there other observers had made tentatives in
the same direction. Boillard had even followed the matter up with
pertinacity, but the world was not ready to listen to him. Now,
however, in the half-decade that followed Broca's announcements,
interest rose to fever-beat, and through the efforts of Broca,
Boillard, and numerous others it was proved that a veritable
centre having a strange domination over the memory of articulate
words has its seat in the third convolution of the frontal lobe
of the cerebrum, usually in the left hemisphere. That part of the
brain has since been known to the English-speaking world as the
convolution of Broca, a name which, strangely enough, the
discoverer's compatriots have been slow to accept.
This discovery very naturally reopened the entire subject of
brain localization. It was but a short step to the inference
that there must be other definite centres worth the seeking, and
various observers set about searching for them. In 1867 a clew
was gained by Eckhard, who, repeating a forgotten experiment by
Haller and Zinn of the previous century, removed portions of the
brain cortex of animals, with the result of producing
convulsions. But the really vital departure was made in 1870 by
the German investigators Fritsch and Hitzig, who, by stimulating
definite areas of the cortex of animals with a galvanic current,
produced contraction of definite sets of muscles of the opposite
side of the body. These most important experiments, received at
first with incredulity, were repeated and extended in 1873 by Dr.
David Ferrier, of London, and soon afterwards by a small army of
independent workers everywhere, prominent among whom were Franck
and Pitres in France, Munck and Goltz in Germany, and Horsley and
Schafer in England. The detailed results, naturally enough, were
not at first all in harmony. Some observers, as Goltz, even
denied the validity of the conclusions in toto. But a consensus
of opinion, based on multitudes of experiments, soon placed the
broad general facts for which Fritsch and Hitzig contended beyond
controversy. It was found, indeed, that the cerebral centres of
motor activities have not quite the finality at first ascribed to
them by some observers, since it may often happen that after the
destruction of a centre, with attending loss of function, there
may be a gradual restoration of the lost function, proving that
other centres have acquired the capacity to take the place of the
one destroyed. There are limits to this capacity for
substitution, however, and with this qualification the
definiteness of the localization of motor functions in the
cerebral cortex has become an accepted part of brain physiology.
Nor is such localization confined to motor centres. Later
experiments, particularly of Ferrier and of Munck, proved that
the centres of vision are equally restricted in their location,
this time in the posterior lobes of the brain, and that hearing
has likewise its local habitation. Indeed, there is every reason
to believe that each form of primary sensation is based on
impressions which mainly come to a definitely localized goal in
the brain. But all this, be it understood, has no reference to
the higher forms of intellection. All experiment has proved
futile to localize these functions, except indeed to the extent
of corroborating the familiar fact of their dependence upon the
brain, and, somewhat problematically, upon the anterior lobes of
the cerebrum in particular. But this is precisely what should be
expected, for the clearer insight into the nature of mental
processes makes it plain that in the main these alleged
"faculties" are not in themselves localized. Thus, for example,
the "faculty" of language is associated irrevocably with centres
of vision, of hearing, and of muscular activity, to go no
further, and only becomes possible through the association of
these widely separated centres. The destruction of Broca's
centre, as was early discovered, does not altogether deprive a
patient of his knowledge of language. He may be totally unable to
speak (though as to this there are all degrees of variation), and
yet may comprehend what is said to him, and be able to read,
think, and even write correctly. Thus it appears that Broca's
centre is peculiarly bound up with the capacity for articulate
speech, but is far enough from being the seat of the faculty of
language in its entirety.
In a similar way, most of the supposed isolated "faculties" of
higher intellection appear, upon clearer analysis, as complex
aggregations of primary sensations, and hence necessarily
dependent upon numerous and scattered centres. Some "faculties,"
as memory and volition, may be said in a sense to be primordial
endowments of every nerve cell--even of every body cell. Indeed,
an ultimate analysis relegates all intellection, in its
primordial adumbrations, to every particle of living matter. But
such refinements of analysis, after all, cannot hide the fact
that certain forms of higher intellection involve a pretty
definite collocation and elaboration of special sensations. Such
specialization, indeed, seems a necessary accompaniment of mental
evolution. That every such specialized function has its
localized centres of co-ordination, of some such significance as
the demonstrated centres of articulate speech, can hardly be in
doubt--though this, be it understood, is an induction, not as yet
a demonstration. In other words, there is every reason to
believe that numerous "centres," in this restricted sense, exist
in the brain that have as yet eluded the investigator. Indeed,
the current conception regards the entire cerebral cortex as
chiefly composed of centres of ultimate co-ordination of
impressions, which in their cruder form are received by more
primitive nervous tissues--the basal ganglia, the cerebellum and
medulla, and the spinal cord.
This, of course, is equivalent to postulating the cerebral cortex
as the exclusive seat of higher intellection. This proposition,
however, to which a safe induction seems to lead, is far afield
from the substantiation of the old conception of brain
localization, which was based on faulty psychology and equally
faulty inductions from few premises. The details of Gall's
system, as propounded by generations of his mostly unworthy
followers, lie quite beyond the pale of scientific discussion.
Yet, as I have said, a germ of truth was there--the idea of
specialization of cerebral functions--and modern investigators
have rescued that central conception from the phrenological
rubbish heap in which its discoverer unfortunately left it
buried.
THE MINUTE STRUCTURE OF THE BRAIN
The common ground of all these various lines of investigations of
pathologist, anatomist, physiologist, physicist, and psychologist
is, clearly, the central nervous system--the spinal cord and the
brain. The importance of these structures as the foci of nervous
and mental activities has been recognized more and more with each
new accretion of knowledge, and the efforts to fathom the secrets
of their intimate structure has been unceasing. For the earlier
students, only the crude methods of gross dissections and
microscopical inspection were available. These could reveal
something, but of course the inner secrets were for the keener
insight of the microscopist alone. And even for him the task of
investigation was far from facile, for the central nervous
tissues are the most delicate and fragile, and on many accounts
the most difficult of manipulation of any in the body.
Special methods, therefore, were needed for this essay, and brain
histology has progressed by fitful impulses, each forward jet
marking the introduction of some ingenious improvement of
mechanical technique, which placed a new weapon in the hands of
the investigators.
The very beginning was made in 1824 by Rolando, who first thought
of cutting chemically hardened pieces of brain tissues into thin
sections for microscopical examination--the basal structure upon
which almost all the later advances have been conducted. Muller
presently discovered that bichromate of potassium in solution
makes the best of fluids for the preliminary preservation and
hardening of the tissues. Stilling, in 1842, perfected the
method by introducing the custom of cutting a series of
consecutive sections of the same tissue, in order to trace nerve
tracts and establish spacial relations. Then from time to time
mechanical ingenuity added fresh details of improvement. It was
found that pieces of hardened tissue of extreme delicacy can be
made better subject to manipulation by being impregnated with
collodion or celloidine and embedded in paraffine. Latterly it
has become usual to cut sections also from fresh tissues,
unchanged by chemicals, by freezing them suddenly with vaporized
ether or, better, carbonic acid. By these methods, and with the
aid of perfected microtomes, the worker of recent periods avails
himself of sections of brain tissues of a tenuousness which the
early investigators could not approach.
But more important even than the cutting of thin sections is the
process of making the different parts of the section visible, one
tissue differentiated from another. The thin section, as the
early workers examined it, was practically colorless, and even
the crudest details of its structure were made out with extreme
difficulty. Remak did, indeed, manage to discover that the brain
tissue is cellular, as early as 1833, and Ehrenberg in the same
year saw that it is also fibrillar, but beyond this no great
advance was made until 1858, when a sudden impulse was received
from a new process introduced by Gerlach. The process itself was
most simple, consisting essentially of nothing more than the
treatment of a microscopical section with a solution of carmine.
But the result was wonderful, for when such a section was placed
under the lens it no longer appeared homogeneous. Sprinkled
through its substance were seen irregular bodies that had taken
on a beautiful color, while the matrix in which they were
embedded remained unstained. In a word, the central nerve cell
had sprung suddenly into clear view.
A most interesting body it proved, this nerve cell, or ganglion
cell, as it came to be called. It was seen to be exceedingly
minute in size, requiring high powers of the microscope to make
it visible. It exists in almost infinite numbers, not, however,
scattered at random through the brain and spinal cord. On the
contrary, it is confined to those portions of the central nervous
masses which to the naked eye appear gray in color, being
altogether wanting in the white substance which makes up the
chief mass of the brain. Even in the gray matter, though
sometimes thickly distributed, the ganglion cells are never in
actual contact one with another; they always lie embedded in
intercellular tissues, which came to be known, following Virchow,
as the neuroglia.
Each ganglion cell was seen to be irregular in contour, and to
have jutting out from it two sets of minute fibres, one set
relatively short, indefinitely numerous, and branching in every
direction; the other set limited in number, sometimes even
single, and starting out directly from the cell as if bent on a
longer journey. The numerous filaments came to be known as
protoplasmic processes; the other fibre was named, after its
discoverer, the axis cylinder of Deiters. It was a natural
inference, though not clearly demonstrable in the sections, that
these filamentous processes are the connecting links between the
different nerve cells and also the channels of communication
between nerve cells and the periphery of the body. The white
substance of brain and cord, apparently, is made up of such
connecting fibres, thus bringing the different ganglion cells
everywhere into communication one with another.
In the attempt to trace the connecting nerve tracts through this
white substance by either macroscopical or microscopical methods,
most important aid is given by a method originated by Waller in
1852. Earlier than that, in 1839, Nasse had discovered that a
severed nerve cord degenerates in its peripheral portions. Waller
discovered that every nerve fibre, sensory or motor, has a nerve
cell to or from which it leads, which dominates its nutrition, so
that it can only retain its vitality while its connection with
that cell is intact. Such cells he named trophic centres.
Certain cells of the anterior part of the spinal cord, for
example, are the trophic centres of the spinal motor nerves.
Other trophic centres, governing nerve tracts in the spinal cord
itself, are in the various regions of the brain. It occurred to
Waller that by destroying such centres, or by severing the
connection at various regions between a nervous tract and its
trophic centre, sharply defined tracts could be made to
degenerate, and their location could subsequently be accurately
defined, as the degenerated tissues take on a changed aspect,
both to macroscopical and microscopical observation. Recognition
of this principle thus gave the experimenter a new weapon of
great efficiency in tracing nervous connections. Moreover, the
same principle has wide application in case of the human subject
in disease, such as the lesion of nerve tracts or the destruction
of centres by localized tumors, by embolisms, or by traumatisms.
All these various methods of anatomical examination combine to
make the conclusion almost unavoidable that the central ganglion
cells are the veritable "centres" of nervous activity to which so
many other lines of research have pointed. The conclusion was
strengthened by experiments of the students of motor
localization, which showed that the veritable centres of their
discovery lie, demonstrably, in the gray cortex of the brain, not
in the white matter. But the full proof came from pathology. At
the hands of a multitude of observers it was shown that in
certain well-known diseases of the spinal cord, with resulting
paralysis, it is the ganglion cells themselves that are found to
be destroyed. Similarly, in the case of sufferers from chronic
insanities, with marked dementia, the ganglion cells of the
cortex of the brain are found to have undergone degeneration. The
brains of paretics in particular show such degeneration, in
striking correspondence with their mental decadence. The position
of the ganglion cell as the ultimate centre of nervous activities
was thus placed beyond dispute.
Meantime, general acceptance being given the histological scheme
of Gerlach, according to which the mass of the white substance of
the brain is a mesh-work of intercellular fibrils, a proximal
idea seemed attainable of the way in which the ganglionic
activities are correlated, and, through association, built up, so
to speak, into the higher mental processes. Such a conception
accorded beautifully with the ideas of the associationists, who
had now become dominant in psychology. But one standing puzzle
attended this otherwise satisfactory correlation of anatomical
observations and psychic analyses. It was this: Since, according
to the histologist, the intercellular fibres, along which
impulses are conveyed, connect each brain cell, directly or
indirectly, with every other brain cell in an endless mesh-work,
how is it possible that various sets of cells may at times be
shut off from one another? Such isolation must take place, for
all normal ideation depends for its integrity quite as much upon
the shutting-out of the great mass of associations as upon the
inclusion of certain other associations. For example, a student
in solving a mathematical problem must for the moment become
quite oblivious to the special associations that have to do with
geography, natural history, and the like. But does histology give
any clew to the way in which such isolation may be effected?
Attempts were made to find an answer through consideration of the
very peculiar character of the blood-supply in the brain. Here,
as nowhere else, the terminal twigs of the arteries are arranged
in closed systems, not anastomosing freely with neighboring
systems. Clearly, then, a restricted area of the brain may,
through the controlling influence of the vasomotor nerves, be
flushed with arterial blood while neighboring parts remain
relatively anaemic. And since vital activities unquestionably
depend in part upon the supply of arterial blood, this peculiar
arrangement of the vascular mechanism may very properly be
supposed to aid in the localized activities of the central
nervous ganglia. But this explanation left much to be desired--in
particular when it is recalled that all higher intellection must
in all probability involve multitudes of widely scattered
centres.
No better explanation was forthcoming, however, until the year
1889, when of a sudden the mystery was cleared away by a fresh
discovery. Not long before this the Italian histologist Dr.
Camille Golgi had discovered a method of impregnating hardened
brain tissues with a solution of nitrate of silver, with the
result of staining the nerve cells and their processes almost
infinitely better than was possible by the methods of Gerlach, or
by any of the multiform methods that other workers had
introduced. Now for the first time it became possible to trace
the cellular prolongations definitely to their termini, for the
finer fibrils had not been rendered visible by any previous
method of treatment. Golgi himself proved that the set of fibrils
known as protoplasmic prolongations terminate by free
extremities, and have no direct connection with any cell save the
one from which they spring. He showed also that the axis
cylinders give off multitudes of lateral branches not hitherto
suspected. But here he paused, missing the real import of the
discovery of which he was hard on the track. It remained for the
Spanish histologist Dr. S. Ramon y Cajal to follow up the
investigation by means of an improved application of Golgi's
method of staining, and to demonstrate that the axis cylinders,
together with all their collateral branches, though sometimes
extending to a great distance, yet finally terminate, like the
other cell prolongations, in arborescent fibrils having free
extremities. In a word, it was shown that each central nerve
cell, with its fibrillar offshoots, is an isolated entity.
Instead of being in physical connection with a multitude of other
nerve cells, it has no direct physical connection with any other
nerve cell whatever.
When Dr. Cajal announced his discovery, in 1889, his
revolutionary claims not unnaturally amazed the mass of
histologists. There were some few of them, however, who were not
quite unprepared for the revelation; in particular His, who had
half suspected the independence of the cells, because they seemed
to develop from dissociated centres; and Forel, who based a
similar suspicion on the fact that he had never been able
actually to trace a fibre from one cell to another. These
observers then came readily to repeat Cajal's experiments. So
also did the veteran histologist Kolliker, and soon afterwards
all the leaders everywhere. The result was a practically
unanimous confirmation of the Spanish histologist's claims, and
within a few months after his announcements the old theory of
union of nerve cells into an endless mesh-work was completely
discarded, and the theory of isolated nerve elements--the theory
of neurons, as it came to be called--was fully established in its
place.
As to how these isolated nerve cells functionate, Dr. Cajal gave
the clew from the very first, and his explanation has met with
universal approval.
In the modified view, the nerve cell retains its old position as
the storehouse of nervous energy. Each of the filaments jutting
out from the cell is held, as before, to be indeed a transmitter
of impulses, but a transmitter that operates intermittently, like
a telephone wire that is not always "connected," and, like that
wire, the nerve fibril operates by contact and not by continuity.
Under proper stimulation the ends of the fibrils reach out, come
in contact with other end fibrils of other cells, and conduct
their destined impulse. Again they retract, and communication
ceases for the time between those particular cells. Meantime, by
a different arrangement of the various conductors, different sets
of cells are placed in communication, different associations of
nervous impulses induced, different trains of thought engendered.
Each fibril when retracted becomes a non-conductor, but when
extended and in contact with another fibril, or with the body of
another cell, it conducts its message as readily as a continuous
filament could do--precisely as in the case of an electric wire.
This conception, founded on a most tangible anatomical basis,
enables us to answer the question as to how ideas are isolated,
and also, as Dr. Cajal points out, throws new light on many other
mental processes. One can imagine, for example, by keeping in
mind the flexible nerve prolongations, how new trains of thought
may be engendered through novel associations of cells; how
facility of thought or of action in certain directions is
acquired through the habitual making of certain nerve-cell
connections; how certain bits of knowledge may escape our memory
and refuse to be found for a time because of a temporary
incapacity of the nerve cells to make the proper connections, and
so on indefinitely.
If one likens each nerve cell to a central telephone office, each
of its filamentous prolongations to a telephone wire, one can
imagine a striking analogy between the modus operandi of nervous
processes and of the telephone system. The utility of new
connections at the central office, the uselessness of the
mechanism when the connections cannot be made, the "wires in use"
that retard your message, perhaps even the crossing of wires,
bringing you a jangle of sounds far different from what you
desire--all these and a multiplicity of other things that will
suggest themselves to every user of the telephone may be imagined
as being almost ludicrously paralleled in the operations of the
nervous mechanism. And that parallel, startling as it may seem,
is not a mere futile imagining. It is sustained and rendered
plausible by a sound substratum of knowledge of the anatomical
conditions under which the central nervous mechanism exists, and
in default of which, as pathology demonstrates with no less
certitude, its functionings are futile to produce the normal
manifestations of higher intellection.