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

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,