A History of Science, vol 4, Henry Smith Williams [best books to read in life txt] 📗
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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 NERVESWhile 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,
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