A History of Science, vol 4 - Henry Smith Williams (epub e ink reader .txt) 📗

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 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 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;