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minute structure of the tissues, ena- cular action gives rise to a waste prod. bles us to form a definite conception uct which has a toxic influence on the of its place of origin. All sensations fibres, affords a clue to the origin of due to the muscles must be conveyed this sensation of fatigue or weariness. by the sensory nerves which belong The muscle-fibres within the spindles to them. These are distinct from the must contract with the rest of the motor nerves, which convey the im- muscle. Although motor nerve fibres pulses that excite contraction. The have not yet been proved to pass withsensory nerves pass down to peculiar in the spindle, yet the effect of stimustructures in the muscles, which have lation passes along a muscular fibre, been thoroughly studied only during throughout its length; and thus the efthe last few years. The nerves termi- fect of the stimulation of the fibre outnate in peculiar long enclosures, taper- side the spindle must extend through ing at each end, and bounded by a defi- the portion within it. The contraction nite wall. These have received the name 'here inevitably gives rise to the same of "muscle-spindles" from their pointed waste product, with the same toxic inends. Into each of these passes a con- fluence. It is only through the sentractile muscular fibre which divides sory nerves of the spindle that the within the spindle; around it the nerve feeling of muscular fatigue can be perends by a peculiar arrangement of its ceived; and we may safely conclude ibrils. These nerves, passing into the that it is through the influence of the spindles, seem to be the only sensory toxic product on the sensory nerve end. nerves of muscles, the only nerves ings that the sensation of muscular which can carry sensations from them. weariness is produced, which so disThe probable explanation of the func tinctly arises in the muscle. These tion of these structures is that the con products of muscular action can escape traction of the contained muscular fibre less readily from within the capsule generates in the nerve fibre, by pres- of the spindle than from the rest of the sure on it, impulses which correspond muscle, and have thus a special opin intensity to the degree of contrac- portunity of acting on the sensory tion, not only of the fibre contained nerves. We can therefore understand within the spindle, but of the whole that the sense of muscular weariness muscle. It bas, therefore, been sug persists so long after exertion has gested that they are of the nature of ceased. It probably lasts longer than "muscle-meters," analogous to the the actual exhaustion of the muscle, meters of an electrical current, which in harmony with our experience that dirert a small portion, and estimate the sensation left by exertion endures it, as an index to the strength of the after the power for renewed exercise whole. These nerves carry all impres- has been regained. Such an influence sions from the muscles which reach may well be salutary, inducing rest the brain. If a muscle is pinched, the until the nutritional capacity for enpain is felt in consequence of the com ergy is fully restored. Hence we can pression of these nerves; and it is also conceive that these muscle-spindles not their compression which gives rise to only are “muscle-meters," informing the intense pain of cramp. It must the brain of the degree of contraction be through them that we receive the of the muscle, but also constitute a sensation of muscular fatigue famil. mechanism having the effect of a dan. iar to every one as a result of over- ger signal, giving warning of the need exertion,

for rest, and keeping the signal up The fact, already noticed, that mus- until the capacity is fully restored.

source of muscular energy. Atoms call "combustion,” and yet analogous. form closer combinations. In the But the process takes place in the live muscle, before it contracts, they are ing tissue; and life shrouds with its held apart by interatomic motion, mi- veil of mystery all that occurs within nute in degree but vast in total amount, its domain. in the elaborate compounds of which T he bindering effect of the products muscle consists, and also in the oxygen of muscular action is peculiarly in. which comes in the blood to the mus- structive. We can understand that cles. When what we call a "stimulus" their removal, even by the agency of acts on the fibres, the atoms composing distilled water, may enable the muscle them suddenly form closer compounds again to respond to a stimulus which by means of the adjacent oxygen. This reaches it; and we can understand that, has a potent attraction for them, to if not removed, these products hinder, which they could not yield until the in all animals, the ability to maintain "stimulus," as it were shaking them, continuous effort. At the same time set them free. Their closer union it must be remembered that another liberates the force which kept them and perhaps the most potent factor in separate. The mystery of muscle is the decay of strength caused by overthat the released energy is so seized exertion is the exhaustion of the eleand united as to make the whole mus- ments of the muscles from which the cle shorten with a force proportioned energy is derived. Their renewal to its size. We do not know how this under the influence of life is speedy, combination of the energy released is ef- but it needs time. The quick repetifected; but we can see its analogy when tion of muscular exertion does not percoal-gas, mixed with air, is exploded mit the living tissues to appropriate, in the cylinder of a gas engine. The in adequate degree, the elements preatoms of coal-gas and of the oxygen sented to them; and thus exhaustion is of the air are kept apart by interatomic induced, which is the essential cause motion, "latent energy"; the spark is of the failure from fatigue, althougb here the stimulus which disturbs the its influence is accompanied, and to balance; closer combination releases the some extent anticipated, by the hinderenergy, and the piston is moved, while ing effect of the products of action. carbonic acid and water result from These facts enable us to understand the union of the atoms previously kept better the sensation of fatigue, alapart.

though their application has hardly yet Between this process and that in the been fully recognized by the students muscle there is a wide and unbridged of the subject. They are of interest, gulf. Yet there is an analogy suffi. also, as an example of the relation ciently close to be instructive. The which one branch of science bears to carbonic acid formed in the gas engine another. Facts which seem isolated would extinguish any light placed in are found to be connected; one disit; through it no other spark could covery may lead to another quite difpass. The combination of atoms in the ferent in character. We all know that muscle which releases energy produces a prominent effect of over-exertion is substances that interfere with a repe- true muscular weariness, a sensation tition of the process. They are toxic experienced in the muscles themselves. to the muscle in so far as they hinder As a feeling, this eludes investigation, the process which causes contraction, as do all our pure sensations; but the They result from chemical union, less discoveries of histology, the branch of direct and less close than what we science which is concerned with the minute structure of the tissues, ena- cular action gives rise to a waste prod. bles us to form a definite conception uct which has a toxic influence on the of its place of origin. All sensations fibres, affords a clue to the origin of due to the muscles must be conveyed this sensation of fatigue or weariness. by the sensory nerves which belong The muscle-fibres within the spindles to them. These are distinct from the must contract with the rest of the motor nerves, which convey the im- muscle. Although motor nerve fibres pulses that excite contraction. The have not yet been proved to pass withsensory nerves pass down to peculiar in the spindle, yet the effect of stimustructures in the muscles, which have lation passes along a muscular fibre, been thoroughly studied only during throughout its length; and thus the efthe last few years. The nerves termi- fect of the stimulation of the fibre outnate in peculiar long enclosures, taper- side the spindle must extend through ing at each end, and bounded by a defi- the portion within it. The contraction nite wall. These have received the name 'here inevitably gives rise to the same of "muscle-spindles" from their pointed waste product, with the same toxic inends. Into each of these passes a con- fluence. It is only through the sentractile muscular fibre which divides sory nerves of the spindle that the within the spindle; around it the nerve feeling of muscular fatigue can be perends by a peculiar arrangement of its ceived; and we may safely conclude fibrils. These nerves, passing into the that it is through the influence of the spindles, seem to be the only sensory toxic product on the sensory nerve endnerves of muscles, the only nerves ings that the sensation of muscular which can carry sensations from them, weariness is produced, which so disThe probable explanation of the func- tinctly arises in the muscle. These tion of these structures is that the con products of muscular action can escape traction of the contained muscular fibre less readily from within the capsule generates in the nerve fibre, by pres of the spindle than from the rest of the sure on it, impulses which correspond muscle, and have thus a special opin intensity to the degree of contrac- portunity of acting on the sensory tion, not only of the fibre contained nerves. We can therefore understand within the spindle, but of the whole that the sense of muscular weariness muscle. It has, therefore, been sug. persists so long after exertion has gested that they are of the nature of ceased. It probably lasts longer than “muscle-meters," analogous to the the actual exhaustion of the muscle, meters of an electrical current, which in harmony with our experience that divert a small portion, and estimate the sensation left by exertion endures it, as an index to the strength of the after the power for renewed exercise whole. These nerves carry all impress has been regained. Such an influence sions from the muscles which reach may well be salutary, inducing rest the brain. If a muscle is pinched, the until the nutritional capacity for enpain is felt in consequence of the com. ergy is fully restored. Hence we can pression of these nerves; and it is also conceive that these muscle-spindles not their compression which gives rise to only are “muscle-meters," informing the intense pain of cramp. It must the brain of the degree of contraction be through them that we receive the of the muscle, but also constitute a sensation of muscular fatigue famil. mechanism having the effect of a dan. iar to every one as a result of over- ger signal, giving warning of the need exertion.

for rest, and keeping the signal up The fact, already noticed, that mus- until the capacity is fully restored.

The facts of fatigue in the brain are ments is charged with these materials, less simple and far more difficut to in the action of the structures of the brain vestigate and understand. We cannot should also be affected. measure and record the power of the We should also remember that the brain as we can that of the muscles. muscles are excited to contraction by Prolonged exertion is known to cause the brain; cerebral action is involved in analogous disability, but it may be all exercise, even in the work of the counteracted for a time by the strange treadmill, and to a greater and wider power of the nerve elements to respond degree in proportion as the muscular to powerful stimulation, at the cost, work needs mental supervision. School however, of ultimate greater collapse. games, for instance, involve a wide Before considering the features of range of brain activity. Most of the brain fatigue, it is interesting to note senses are called into action; comparithe connection between muscular exer- son and judgment are needed, and the tion and cerebral activity. Mosso sources of nerve activity are more or mentions that birds at the end of a less exhausted. The double influence long migratory flight, when utterly of games, the demand on brain and fatigued, seem unable to see, or, at muscle, and the hindrance to each any rate, to perceive the nature of which results from the products of what is before them. They will fly their action, enable us readily to comagainst a house or rock or other ob- prehend the failure of the cricketer's ject, and fall dead.

dexterity after a long day's play, and Apparently the effect of the exces. the inability of the schoolboy to work sive muscular effort is to lessen or well after hard exertion. Indeed this abolish the sensory power of the cere- lesson is one of great importance. It is bral centres. It is said that Alpine not reasonable to expect mental work climbers sometimes remember very lit. to be well done after exhausting mustle of the incidents of the last part of cular exercise. a fatiguing ascent; in this case, how Of all the processes of nature that ever, other causes, as the state of the we can study, perhaps the most mysair, may co-operate. But it is easy to terious are those chemical changes understand that prolonged muscular which occur under the influence of life. effort may cause grave interference There are very few rifts in the cloud with brain function. The products of that envelops and obscures this occult muscular activity, which have such a interchange of material, and the energy restraining infuence on the muscular which waits upon it. At this we have fibres, pass readily into the blood, and already glanced in speaking of musreach all parts of the body, including cular action. Some investigators bethe brain. The influence they have on lieve that all chemical change, when the muscle is doubtless also exerted on thoroughly known, may be found to the cerebral structures. The muscle be of quite other nature, and to consubstance more nearly resembles nerve sist really of electrical processes. But tissue than does any other substance whatever discernment may come, with in the body; but that tissue exceeds the growth of knowledge, concerning muscle in sensitiveness to harmful sub physical processes, these have little restances. We can understand that the lation to vital action. Electricity in remuscular products should have a lation to life is as mysterious, as dim, similar, and perhaps greater, effect on as any other form of force. For defithe cerebral tissue, and that when the nite knowledge we shall have apparentplasma which bathes the nerve ele- ly to wait, and to wait long-perhaps

until we have learned whether there is cially by such as are formed by the such a thing as matter at all.

action of the nerve tissue itself. Meanwhile these processes which go As with the muscle, so with the neron under the influence of life are recog. vous elements; their action generates nizable by us only in dimly discerned their own products hindering activity. outline, and in their definite results. These result from all action; in slight We can perceive that all action of the amount they have little influence, in nerve elements in the brain is attended greater amount they restrain. After by chemical change, just as is the great muscular exertion the influences activity of the muscular substance. of the two products, those of muscular The molecules composing them break and of nerve action, coincide. The up; atoms pass away in lower com- greater delicacy of the higher strucpounds, just as from the muscles. tures makes them extremely sensitive Energy, previously held latent, is re- to such toxic agents. The nerve subleased as nerve force, the mysterious stance is susceptible to many organic form of energy which traverses the poisons; and this susceptibility varies nerve fibres and stimulates the mus- even in different parts, between which cles to energetic contraction. Of its na- our coarse methods of analysis can find ture we know really nothing. We must no difference. Strychnia, which excites conceive it to be a form of energy, the spinal cord to intense activity, has most slight in absolute degree, but no action on the pupil of the eye; but most potent in its effect on the suscep- the pupil is widely dilated by the tible structures. It is doubtful whether stimulus of a small quantity of atropin, any measure of force is sufficiently even by the five-thousandth part of a delicate to express the degree of that grain, which has no influence on the which, passing down a nerve, will spinal cord. . excite a large muscle to strong con. But in the nerve structures, as in the traction. It may be no exaggeration muscles, the exhaustion of the capacity to say that nerve force which, ex- for action must be regarded as the pressed in terms of motion, would raise chief cause of the inability which a milligramme (say a grain of salt) a brain-work itself entails. We cannot millimetre high (i.e. through a space here measure the relative effect of the equal to the thickness of a postcard) two influences the exhaustion of the would suffice to excite a muscular con- tissue power, which is the expression traction that would raise a pound and result of true brain-fatigue, and weight through a foot. So great is the its hindrance by the products of its disproportion between the two-be- own action; but one tangible proof of tween the energy of the excitant and brain-exhaustion is afforded by the the energy liberated through its in fact that brain-work lessens the power fluence.

of maintaining muscular action. This This nerve force is the most deli. fact is clearly shown by some of cate of all the dynamical processes Mosso's observations with his ergoof life. It is easy to conceive that the graph. The strength of successive structures that evolve it should be movements of the finger diminishes equally delicate in equilibrium, most much more rapidly after energetic menreadily excited, and also most readily tal labor than under normal conditions. hindered. They may thus be hindered This can only be ascribed to a lesby the presence of any substances that sened degree of the nerve energy which have a restraining effect, such as re- stimulates the muscles. It is not easy sult from muscular activity, and espe. to understand how purely mental work

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