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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 psychophysics. 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 psychophysics. 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 PSYCHOLOGYHardly 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;
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