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

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 “vasomotor” 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.

PSYCHOPHYSICS

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