minute structure of the tissues, enables us to form a definite conception of its place of origin. All sensations due to the muscles must be conveyed by the sensory nerves which belong to them. These are distinct from the motor nerves, which convey the impulses that excite contraction. The sensory nerves pass down to peculiar structures in the muscles, which have been thoroughly studied only during the last few years. The nerves terminate in peculiar long enclosures, tapering at each end, and bounded by a definite wall. These have received the name of "muscle-spindles" from their pointed ends. Into each of these passes a contractile muscular fibre which divides within the spindle; around it the nerve ends by a peculiar arrangement of its fibrils. These nerves, passing into the spindles, seem to be the only sensory nerves of muscles, the only nerves which can carry sensations from them. The probable explanation of the function of these structures is that the contraction of the contained muscular fibre generates in the nerve fibre, by pressure on it, impulses which correspond in intensity to the degree of contraction, not only of the fibre contained within the spindle, but of the whole muscle. It has, therefore, been suggested that they are of the nature of "muscle-meters," analogous to the meters of an electrical current, which divert a small portion, and estimate it, as an index to the strength of the whole. These nerves carry all impressions from the muscles which reach the brain. If a muscle is pinched, the pain is felt in consequence of the compression of these nerves; and it is also their compression which gives rise to the intense pain of cramp. It must be through them that we receive the sensation of muscular fatigue familiar to every one as a result of overexertion.

The fact, already noticed, that mus

cular action gives rise to a waste product which has a toxic influence on the fibres, affords a clue to the origin of this sensation of fatigue or weariness. The muscle-fibres within the spindles must contract with the rest of the muscle. Although motor nerve fibres have not yet been proved to pass within the spindle, yet the effect of stimulation passes along a muscular fibre, throughout its length; and thus the effect of the stimulation of the fibre outside the spindle must extend through the portion within it. The contraction 'here inevitably gives rise to the same waste product, with the same toxic influence. It is only through the sensory nerves of the spindle that the feeling of muscular fatigue can be perceived; and we may safely conclude that it is through the influence of the toxic product on the sensory nerve endings that the sensation of muscular weariness is produced, which so distinctly arises in the muscle. These products of muscular action can escape less readily from within the capsule of the spindle than from the rest of the muscle, and have thus a special opportunity of acting on the sensory nerves. We can therefore understand that the sense of muscular weariness persists so long after exertion has ceased. It probably lasts longer than the actual exhaustion of the muscle, in harmony with our experience that the sensation left by exertion endures after the power for renewed exercise has been regained. Such an influence may well be salutary, inducing rest until the nutritional capacity for energy is fully restored. Hence we can conceive that these muscle-spindles not only are "muscle-meters," informing the brain of the degree of contraction of the muscle, but also constitute a mechanism having the effect of a danger signal, giving warning of the need for rest, and keeping the signal up until the capacity is fully restored.

Atoms

source of muscular energy. form closer combinations. In the 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.

The

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. 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

minute structure of the tissues, enables us to form a definite conception of its place of origin. All sensations due to the muscles must be conveyed by the sensory nerves which belong to them. These are distinct from the motor nerves, which convey the impulses that excite contraction. The sensory nerves pass down to peculiar structures in the muscles, which have been thoroughly studied only during the last few years. The nerves terminate in peculiar long enclosures, tapering at each end, and bounded by a definite wall. These have received the name of "muscle-spindles" from their pointed ends. Into each of these passes a contractile muscular fibre which divides within the spindle; around it the nerve ends by a peculiar arrangement of its fibrils. These nerves, passing into the spindles, seem to be the only sensory nerves of muscles, the only nerves which can carry sensations from them. The probable explanation of the function of these structures is that the contraction of the contained muscular fibre generates in the nerve fibre, by pressure on it, impulses which correspond in intensity to the degree of contraction, not only of the fibre contained within the spindle, but of the whole muscle. It has, therefore, been suggested that they are of the nature of "muscle-meters," analogous to the meters of an electrical current, which divert a small portion, and estimate it, as an index to the strength of the whole. These nerves carry all impressions from the muscles which reach the brain. If a muscle is pinched, the pain is felt in consequence of the compression of these nerves; and it is also their compression which gives rise to the intense pain of cramp. It must be through them that we receive the sensation of muscular fatigue famil. iar to every one as a result of overexertion.

The fact, already noticed, that mus

cular action gives rise to a waste product which has a toxic influence on the fibres, affords a clue to the origin of this sensation of fatigue or weariness. The muscle-fibres within the spindles must contract with the rest of the muscle. Although motor nerve fibres have not yet been proved to pass within the spindle, yet the effect of stimulation passes along a muscular fibre, throughout its length; and thus the effect of the stimulation of the fibre outside the spindle must extend through the portion within it. The contraction here inevitably gives rise to the same waste product, with the same toxic influence. It is only through the sensory nerves of the spindle that the feeling of muscular fatigue can be perceived; and we may safely conclude that it is through the influence of the toxic product on the sensory nerve endings that the sensation of muscular weariness is produced, which so distinctly arises in the muscle. These products of muscular action can escape less readily from within the capsule of the spindle than from the rest of the muscle, and have thus a special opportunity of acting on the sensory nerves. We can therefore understand that the sense of muscular weariness persists so long after exertion has ceased. It probably lasts longer than the actual exhaustion of the muscle, in harmony with our experience that the sensation left by exertion endures after the power for renewed exercise has been regained. Such an influence may well be salutary, inducing rest until the nutritional capacity for energy is fully restored. Hence we can conceive that these muscle-spindles not only are "muscle-meters," informing the brain of the degree of contraction of the muscle, but also constitute a mechanism having the effect of a danger signal, giving warning of the need for rest, and keeping the signal up until the capacity is fully restored.

The facts of fatigue in the brain are less simple and far more difficut to investigate and understand. We cannot measure and record the power of the brain as we can that of the muscles. Prolonged exertion is known to cause analogous disability, but it may be counteracted for a time by the strange power of the nerve elements to respond to powerful stimulation, at the cost, however, of ultimate greater collapse. Before considering the features of brain fatigue, it is interesting to note the connection between muscular exertion and cerebral activity. Mosso mentions that birds at the end of a long migratory flight, when utterly fatigued, seem unable to see, or, at any rate, to perceive the nature of what is before them. They will fly against a house or rock or other object, and fall dead.

Apparently the effect of the excessive muscular effort is to lessen or abolish the sensory power of the cerebral centres. It is said that Alpine climbers sometimes remember very little of the incidents of the last part of a fatiguing ascent; in this case, however, other causes, as the state of the air, may co-operate. But it is easy to understand that prolonged muscular effort may cause grave interference with brain function. The products of muscular activity, which have such a restraining influence on the muscular fibres, pass readily into the blood, and reach all parts of the body, including the brain. The influence they have on the muscle is doubtless also exerted on the cerebral structures. The muscle substance more nearly resembles nerve tissue than does any other substance in the body; but that tissue exceeds muscle in sensitiveness to harmful substances. We can understand that the muscular products should have a similar, and perhaps greater, effect on the cerebral tissue, and that when the plasma which bathes the nerve ele

ments is charged with these materials, the action of the structures of the brain should also be affected.

We should also remember that the muscles are excited to contraction by the brain; cerebral action is involved in all exercise, even in the work of the treadmill, and to a greater and wider degree in proportion as the muscular work needs mental supervision. School games, for instance, involve a wide range of brain activity. Most of the senses are called into action; comparison and judgment are needed, and the sources of nerve activity are more or less exhausted. The double influence of games, the demand on brain and muscle, and the hindrance to each which results from the products of their action, enable us readily to comprehend the failure of the cricketer's dexterity after a long day's play, and the inability of the schoolboy to work well after hard exertion. Indeed this lesson is one of great importance. It is not reasonable to expect mental work to be well done after exhausting muscular exercise.

Of all the processes of nature that we can study, perhaps the most mysterious are those chemical changes which occur under the influence of life. There are very few rifts in the cloud that envelops and obscures this occult interchange of material, and the energy which waits upon it. At this we have already glanced in speaking of muscular action. Some investigators believe that all chemical change, when thoroughly known, may be found to be of quite other nature, and to consist really of electrical processes. But whatever discernment may come, with the growth of knowledge, concerning physical processes, these have little relation to vital action. Electricity in relation to life is as mysterious, as dim, as any other form of force. For definite knowledge we shall have apparently to wait, and to wait long-perhaps

until we have learned whether there is such a thing as matter at all.

Meanwhile these processes which go on under the influence of life are recognizable by us only in dimly discerned outline, and in their definite results. We can perceive that all action of the nerve elements in the brain is attended by chemical change, just as is the activity of the muscular substance. The molecules composing them break up; atoms pass away in lower compounds, just as from the muscles. Energy, previously held latent, is released as nerve force, the mysterious form of energy which traverses the nerve fibres and stimulates the muscles to energetic contraction. Of its nature we know really nothing. We must conceive it to be a form of energy, most slight in absolute degree, but most potent in its effect on the susceptible structures. It is doubtful whether any measure of force is sufficiently delicate to express the degree of that which, passing down a nerve, will excite a large muscle to strong contraction. It may be no exaggeration to say that nerve force which, expressed in terms of motion, would raise a milligramme (say a grain of salt) a millimetre high (i.e. through a space equal to the thickness of a postcard) would suffice to excite a muscular contraction that would raise a pound weight through a foot. So great is the disproportion between the two-between the energy of the excitant and the energy liberated through its influence.

This nerve force is the most delicate of all the dynamical processes of life. It is easy to conceive that the structures that evolve it should be equally delicate in equilibrium, most readily excited, and also most readily hindered. They may thus be hindered by the presence of any substances that have a restraining effect, such as result from muscular activity, and espe

cially by such as are formed by the action of the nerve tissue itself.

As with the muscle, so with the nervous elements; their action generates their own products hindering activity. These result from all action; in slight amount they have little influence, in greater amount they restrain. After great muscular exertion the influences of the two products, those of muscular and of nerve action, coincide. The greater delicacy of the higher structures makes them extremely sensitive to such toxic agents. The nerve substance is susceptible to many organic poisons; and this susceptibility varies even in different parts, between which our coarse methods of analysis can find no difference. Strychnia, which excites the spinal cord to intense activity, has no action on the pupil of the eye; but the pupil is widely dilated by the stimulus of a small quantity of atropin, even by the five-thousandth part of a grain, which has no influence on the spinal cord.

But in the nerve structures, as in the muscles, the exhaustion of the capacity for action must be regarded as the chief cause of the inability which brain-work itself entails. We cannot here measure the relative effect of the two influences-the exhaustion of the tissue power, which is the expression and result of true brain-fatigue, and its hindrance by the products of its own action; but one tangible proof of brain-exhaustion is afforded by the fact that brain-work lessens the power of maintaining muscular action. This fact is clearly shown by some of Mosso's observations with his ergograph. The strength of successive movements of the finger diminishes much more rapidly after energetic mental labor than under normal conditions. This can only be ascribed to a lessened degree of the nerve energy which stimulates the muscles. It is not easy to understand how purely mental work