Page images
PDF
EPUB
[merged small][ocr errors][merged small][merged small][merged small][merged small]

Fatigue is a phase of life to which few are strangers. That which the word denotes is an experience only too familiar to most persons, but in varied character and degree. It is a feature of perfect health, and yet is a link with disease, since it is produced with undue readiness in morbid states, and in some it constitutes a conspicuous symptom. Not only is it varied in its manifestation, but it has many-sided relations; and some of these involve considerable scientific interest. As a result of activity in the normal state, it is a part of physiology, the study of the living body in health; and as such it has been recently made the subject of much research, which has resulted in discoveries of considerable importance. It is a difficult subject for investigation, for reasons which will presently be mentioned; and it is curious that the study it has received has been chiefly at the hands of Italians. That nation has shared conspicuously the impulse to scientific re

*1"Fatigue." By A. Mosso, Professor of Physiology in the University of Turin. Translated by Margaret Drummond, M.A., and W. B. Drummond M.B., London: Swan Sonnenschein, 1804.

2"Weariness." The Rede Lecture, delivered in the University of Cambridge, 1893.

search that has recently affected all civilized peoples, and has extended even to the state that is now so prominent in the eyes of the world-Japan. Italy has grand traditions to inspire her; and the degree in which she excelled in the study of life three centuries ago may have inspired the noteworthy work in physiology which her sons have lately achieved.

Contagion is not confined to disease; it is manifested also in tendencies of thought and work. The special study that has been given by Italians to the subject of fatigue seems chiefly due to the fact that one of their best known physiologists, Professor Mosso, has made it for many years a favorite subject of investigation. He has published the results of his work in many papers, and has condensed them in a small volume designed for popular consumption, which has been translated into English. But fatigue is largely a feeling, a fact of sensation; and our meagre knowledge of the processes which un

By Professor Sir Michael Foster, K.C.B., "Nineteenth Century," September 1893.

3 "Remarks on Replies by Teachers to Questions respecting Mental Fatigue." By Francis Galton, F.R.S., "Journal of the Anthropological Institute," vol. XVIII, 1889.

derlie its sensory phenomena was admirably described by Sir Michael Foster in his Rede Lecture on "Weariness," given before the University of Cambridge. This lecture is a remarkable example of the use of simple, apt language to describe recondite scientific facts.

It is curious that a fact of life so keenly and generally felt as is fatigue should have received systematic study only in recent years. The cause of its neglect becomes perceptible when we discern how little even the latest research can teach us of the nature of weariness, how little science can add to that which every one knows by experience. We may find an inkling of this in the words we use to designate the condition. The word, "fatigue” and all its synonyms, "tiredness," "weariness," "exhaustion," and the like, are positive terms. They are designations of the definite sensation which attends over-exertion. Yet, when we think of fatigue and exhaustion, we think of the inability for further exertion which accompanies the sensation quite as much as of the sensation itself. There are thus two sides to our perception of fatigue a positive side, the sensation of weariness, and a negative side, the diminished power of exertion. Each is prominent in our thoughts. When we speak of being "tired," we mean, generally, that we cannot go on with the effort; yet only the definite sensation finds expression in our words. "Exhaustion" is the nearest approach to a distinctly negative term we use, but this is really positive. The fact is, indeed, an illustration of the way in which all sensations dominate our thoughts and the words which convey them. Our feelings are the most definite realities to our consciousness; they govern our language and often exert a strong influence on more than our words.

Unfortunately for science, feeling or

sensation for the most part eludes our grasp. The actual sensory functions of the nerves can be tested-the sensitiveness of the skin to touch or pain, of the auditory nerve to hearing, of the eye to light and color; but the multitudinous sensations of which the brain may be conscious elude the methods of scientific research even in its latest elaboration. They cannot be described in words, for our feelings altogether transcend the capacities of language; and only similes can be used, which mislead rather than inform. To this class of uncomprehended sensations belong those which are caused by overexertion. The "feelings" of fatigue constitute an obstacle to exertion often insuperable, but their purely subjective nature makes their scientific investigation almost impossible. That which is only felt cannot be recorded, and eludes the precise observation that is necessary for accurate study.

Hence the only aspect of fatigue which is open to research is its negative nature, the diminished power which results from over-exertion. The fact that strength is lessened by continued effort, even in moderate degree, is a matter of familiar observation. Animal life sometimes affords us striking examples; and one pertinent instance is the utter exhaustion of migratory birds when they have had to fly against an adverse wind. Birds vary much in their power of long flight; and the distance travelled by swallows and swifts is less marvellous than that covered by birds such as quails; which seem to have no great strength of wing, and yet are migratory. On reaching land they are often scarcely able to move; and many fail, simply from exhaustion, to reach the shore. Carrier pigeons, which have flown long distances, present the same symptoms of exhaustion; and the effect of overwork has been found by Mosso to be shown in them by increased tempera

ture, and even by an altered color of the muscles which move the wings.

But such observations are not definite enough for modern science. The influence of muscular exertion can be observed, measured, and recorded with precision. The aid which mechanics have given to the study of life is remarkable. It is not a jest, but a sober fact, to say that the science of physiology has been revolutionized by a revolving cylinder. Moved by clockwork at varying speeds, this simple apparatus has opened up a range of precise observation which has almost transformed the investigation of vital phenomena. To those who know anything of physiological science, the use of such an apparatus is so familiar that they have perhaps never thought of what physiology would be without it. But, if the knowledge gained by its means could be eliminated, that which remains would be little more than was perceived fifty years ago, except in the domain of the chemistry of life. It is, indeed, strange how deep a debt physiology owes to simple mechanics.

For those who are not familiar with practical physiology it may be said that the cylinder is covered with paper blackened by the soot of such a smoke as is given off by burning camphor. On this black surface, as the cylinder revolves, a white line is traced by a point attached to a lever; this lever magnifies, perhaps a dozen times, the movement to be recorded. The cylinder, about two feet in circumference, may be made to revolve once a minute or oftener, even once in a few seconds; and any process which can cause a movement can thus be made to record itself in the variation of a definite line. Two processes can be made to produce a record at the same time; and thus the interval which separates them is revealed, although it may be far smaller than could be distinguished by the eye. If twenty-four inches of paper pass

beneath the writing points in a second, a difference in space of one tenth of an inch will correspond to one 240th of a second, a period far too short to be discerned by the eye. Moreover, to aid the comparison of space and time, the science of acoustics is called into service. Every tuning-fork vibrates a definite number of times in a second. It may be made to record its vibrations on the cylinder while the observations are made; and thus an absolute measure of time is written simultaneously on the blackened surface, which indicates, with perfect certainty, the interval of time to which a given space corresponds. By this means facts have been ascertained regarding every process of the animal body which can produce a movement. Even the rate of the transmission of a nerve-impulse has been measured. Although a touch, and the feeling it produces, seem simultaneous to the most careful observer, they are found to be separated by a large fraction of a second.

By an ingenious contrivance, which he calls the "ergograph," Mosso has recorded the strength exerted by the muscles which bend one of the fingers. If a weight is attached to the instrument, the exhaustion of the muscles on successive contractions can be ascertained and indicated by the height to which the weight is raised; and this is recorded by the tracing of the lever.

The gradual diminution of the strength which can be exerted, slow or quick, according to the various conditions of the body, is presented in a large series of diagrams in Mosso's book. The diminution occurs equally, whether the muscles are set in action by the will or are stimulated by an electrical shock to the nerves. The features of their exhaustion have been studied more completely by experiments on animals, in which, indeed, Mosso was long anticipated. The frog is a convenient

1

agent for such observations, because it will go on living for a long time after being killed. The statement may seem somewhat Hibernian; but the division of the spinal cord from the brain does not end life at once, as it would in a higher animal. The heart continues to beat and the muscles to contract, although the brain cannot act on the body, and no sensation can reach the brain. This fact is extremely convenient for physiologists. They can study many of the facts of life, and yet know that they are causing no pain, and that the will of the subject does not influence the facts they ob

serve.

One of these facts is perhaps the most important that has been ascertained regarding the exhaustion which accompanies what we call fatigue. If the muscles of the leg of a frog, thus deprived of conscious feeling, are stimulated by electricity so as to cause contractions in quick succession, these steadily diminish in strength. The height to which the attached lever rises diminishes rapidly, as is shown by the tracing which it records on the cylinder. The diminution goes on until the contraction no longer occurs. The same electrical current passed through the nerve, which at first produced energetic movement, no longer causes contraction in the muscle. If, then, the artery of the limb is divided and distilled water is injected until it flows out freely from the veins, muscular contractions can again be obtained, and they continue for a short time. The significance of this fact is clear. Distilled water cannot renew the contractile elements of the muscle. All that it can do is, so to speak, to wash out the muscle. Hence it is certain that the cessation of the contractions, under rapid stimulation, is due not only to exhaustion of the muscle, but to the presence of something which

hinders the response and can be removed by simple irrigation.

This fact makes us consider more closely what occurs in the muscle when it contracts. The manner in which these wonderful fibres of the muscular substance shorten and widen under a stimulus, is a marvel of which we understand but little. An impulse comes to them through the nerves-an impulse which may be produced by the will or generated by an electrical stimulation of the nerve; and the fibres with one accord become broader and shorter, drawing together the ends of the muscle and thus moving whatever is mobile to which the muscle is attached. By this simultaneous action, united in the vast number of fibres that compose a muscle, these microscopic bands exert a force that is marvellous. The single fibres are far too small to be visible to the naked eye, yet they are so disposed as to pass into synchronous contraction, and furnish a striking example of the way in which number replaces size. Indeed, multiplicity is size. Yet it needs an effort to comprehend that a collection of fibres, each comparable in dimension to a gossamer thread, just visible as it floats in the sunbeams, should be capable of raising half a hundred-weight or

more.

Whence comes the energy thus exerted? The question may be unnecessary; the answer may be well known. Yet upon it depends in part our explanation of fatigue. That force which moves a weight cannot arise de novo is now a matter of common knowledge. It can only be produced by being transformed, by undergoing a change in its relation to matter. Radium, indeed, gives a startling shock to our conceptions, but we are beginning to perceive that it does not really disarrange our old ideas, whatever it may add to them. We can still trust our old conclusions as to the

[blocks in formation]

muscle, before it contracts, they are held apart by interatomic motion, minute in degree but vast in total amount, in the elaborate compounds of which muscle consists, and also in the oxygen which comes in the blood to the muscles. When what we call a "stimulus" acts on the fibres, the atoms composing them suddenly form closer compounds by means of the adjacent oxygen. This has a potent attraction for them, to which they could not yield until the "stimulus," as it were shaking them, set them free. Their closer union liberates the force which kept them separate. The mystery of muscle is that the released energy is so seized and united as to make the whole muscle shorten with a force proportioned to its size. We do not know how this combination of the energy released is effected; but we can see its analogy when coal-gas, mixed with air, is exploded in the cylinder of a gas engine. The atoms of coal-gas and of the oxygen of the air are kept apart by interatomic motion, "latent energy"; the spark is here the stimulus which disturbs the balance; closer combination releases the energy, and the piston is moved, while carbonic acid and water result from the union of the atoms previously kept apart.

Between this process and that in the muscle there is a wide and unbridged gulf. Yet there is an analogy sufficiently close to be instructive. The carbonic acid formed in the gas-engine would extinguish any light placed in it; through it no other spark could pass. The combination of atoms in the muscle which releases energy produces substances that interfere with a repetition of the process. They are toxic to the muscle in so far as they hinder the process which causes contraction. They result from chemical union, less direct and less close than what we

call "combustion," and yet analogous. But the process takes place in the living tissue; and life shrouds with its veil of mystery all that occurs within its domain.

The hindering effect of the products of muscular action is peculiarly instructive. We can understand that their removal, even by the agency of distilled water, may enable the muscle again to respond to a stimulus which reaches it; and we can understand that, if not removed, these products hinder, in all animals, the ability to maintain continuous effort. At the same time it must be remembered that another and perhaps the most potent factor in the decay of strength caused by overexertion is the exhaustion of the elements of the muscles from which the energy is derived. Their renewal under the influence of life is speedy, but it needs time. The quick repetition of muscular exertion does not permit the living tissues to appropriate, in adequate degree, the elements presented to them; and thus exhaustion is induced, which is the essential cause of the failure from fatigue, although its influence is accompanied, and to some extent anticipated, by the hindering effect of the products of action.

These facts enable us to understand better the sensation of fatigue, although their application has hardly yet been fully recognized by the students of the subject. They are of interest, also, as an example of the relation which one branch of science bears to another. Facts which seem isolated are found to be connected; one discovery may lead to another quite different in character. We all know that a prominent effect of over-exertion is true muscular weariness, a sensation experienced in the muscles themselves. As a feeling, this eludes investigation, as do all our pure sensations; but the discoveries of histology, the branch of science which is concerned with the

« PreviousContinue »