Muscle transforms the energy supplied to it by the blood into mechanical work. It is doubtful whether any hypothesis as to structure will help us to an understanding of the way in which this transformation is effected. Explanations are seductive, but all attempts at explaining the connection between molecular change and change in shape must be viewed with suspicion. It is quite clear that muscle as a motor is not to be compared with any form of motor with which we are acquainted. It is also clear that the theory of muscle must be applicable to all its varieties—striped, cardiac, and plain. It must cover the alterations in form of an amœba and the streaming movements of protoplasm within a vegetable cell. Probably it must extend farther, and cover the discharge of electricity by an electric organ and the emission of light by the lamp of a firefly. We are on ground so treacherous that we are not sure whether, in crossing it, we may lean with confidence on the laws of thermodynamics; and doubt as to the applicability of these laws to living tissue almost upsets one’s mental balance. Until we have evidence to the contrary, we are bound to exclude such a misgiving from our minds. If we allow it to influence us at all, it is merely to the extent of causing us to hesitate to assume that the explanation of muscular contraction can be based upon an analogy between muscle and any known mechanical contrivances for generating power, not even excluding apparatus designed for the purpose of measuring osmotic force.
If living muscle is frozen, pounded with snow containing 0·6 per cent. of sodic chloride, and placed upon a filter, a fluid plasma passes through the filter as the mixture thaws. Like blood-plasma, it clots spontaneously—without, however, so far as is known the intervention of a ferment.
All muscles become rigid after death, owing to the coagulation of their plasma. It used to be thought that contraction was a stage towards rigidity—a stage from which muscle, so long as it is alive, recovers. This view was based upon the fact that exhausted muscle—such, for example, as that of a hare which has been coursed—becomes rigid much sooner than rested muscle. But this phenomenon has a different explanation. The setting of muscle in rigor mortis is due to the development of lactic acid (one of the waste products of active muscle). The more there is of this ready formed at the time of death, the more quickly does coagulation of muscle-plasma occur. The formation of lactic acid is due to deficiency of oxygen. So long as muscle obtains as much oxygen as it wants, its metabolism is complete. The oxidized products which it loses are water and carbonic acid. This is true also of the changes which occur after death. If a strip of muscle is hung in an atmosphere of oxygen, it forms no lactic acid, and it does not become rigid. If, on the other hand, the supply of oxygen has run short before death occurred, rigor mortis sets in very quickly. A dead frog takes a long while in becoming rigid, and its rigidity is transient. Until the moment of death the frog is taking up oxygen through its lungs, and even after death it probably takes it, as it does when it is alive, through the skin. A fish becomes rigid very quickly. For some time after it is caught it continues to live; but, being unable to breathe in air, every molecule of oxygen which was in its body when it left the water is used up before it dies.
In the human body rigor mortis usually sets in from two to four hours after death, and lasts about two days; but both the rapidity of its appearance and its duration depend upon various circumstances. As muscles become rigid they contract, moving the limbs, and the shortening is more extensive than mere coagulation of muscle-plasma would account for. It is evident that a process similar to functional contraction precedes coagulation. Many a watcher in the chamber of death has been startled by the shaking of the bed. Even a sound resembling a sigh may be caused by contraction of the muscles of the chest. Placing his hand over the region of the heart, the attendant finds the body warmer than it was when life became extinct, for much oxidation has since taken place.
What chemical changes occur in muscle when it contracts? What is the chemical source of its power? Carbonic acid is given off. This is the only product which we can collect and measure; but it is taken for granted that hydrogen atoms also combine with oxygen, forming water. There is no reason for thinking that nitrogen is removed from the molecules of its protoplasm with any greater rapidity during the activity of muscle than when it is quiescent ([cf. p. 212]). Is the oxidation immediate and complete, or does it occur in stages? For many years attention has been directed to lactic acid, partly because this substance is found in muscle which has been made to contract under experimental conditions, partly because, on theoretical grounds, glycogen (animal starch) is looked upon as the most important of muscle-foods. Lactic acid—C₃H₆O₃—has the same percentage composition as glycogen—C₆H₁₂O₆. Its formation from glycogen merely involves a rearrangement of atoms. It has been supposed that lactic acid is formed in the first instance, and then, if the supply of oxygen be sufficient, oxidized to carbonic acid and water. But this hypothesis may be resisted on various grounds. Undoubtedly, lactic acid appears when oxygen is deficient. Under all circumstances and in all tissues a certain amount of it is formed. There are reasons for thinking that it carries away the nitrogen which is wasted, as lactamide. But it does not follow that under normal conditions, when muscle is abundantly supplied with blood, lactic acid appears in any greater quantity during activity than during rest. The hypothesis is due to the misconception which we have already endeavoured to correct. It is difficult to get away from the steam-engine analogy. A steam-engine is made of iron and brass. These materials are subject to wear and tear; but they are not the source of its power. Its power is due to the combustion of fuel. Muscle, physiologists formerly said, is made of protoplasm. This wears down when it works, setting free creatin and other nitrogenous débris. Its fuel is glycogen. This is not the way, however, in which the matter is now regarded. Protoplasm is not the machine only, but also the source of power. Glycogen is not burnt in a framework of protoplasm. When muscle contracts, protoplasm casts out CO₂ and H₂O. Glycogen is the food readiest to restore to it the atoms which it has lost.
Another consideration opposed to the hypothesis of the conversion of glycogen into lactic acid is the uselessness of such a transformation from a physical point of view. The stability of the atoms of C₃H₆O₃ is so little greater than that of the atoms of C₆H₁₂O₆ that practically no energy is set free when the one substance changes into the other. We cannot, however, overlook the fact that the formation of acid may be a means of profoundly altering the state of the colloid substances dissolved in cell-juice. The casein of milk coagulates when milk turns sour. The neutralization of a faintly alkaline solution of a protein (and muscle is faintly alkaline) will throw it out of solution. The appearance of lactic acid may be intimately associated with movement of protoplasm, and yet the change of glycogen into lactic acid not be the source of the energy which muscle expends.
Fatigue.—For its continued activity muscle needs an adequate supply of food and oxygen. If the blood which distributes food is circulating properly, and the liver, the great depot of food, is well stored, fresh supplies are brought to the muscles as they are needed. There are muscles—those of the eye and of the heart, for example—which never become exhausted. However continuous their activity, they take food from the blood as rapidly as they waste it; a statement which, perhaps, needs qualifying by the addition, “so long as the work exacted of them is such as may be reasonably expected.” If, in a picture-gallery, one keeps the eyes elevated for an hour or more a headache follows. Our eye-muscles have taken over their duties on the understanding that we look down or straight forwards far more often than we look up. If a long-sighted child is required to focus his eyes upon a printed page without the aid of spectacles, not headache merely, but actual disease of the brain, may be the result. The ciliary muscle within the eyeball, which effects accommodation of the eye for near objects, is unduly strained. Even the use of our modern type, with its vertical height greater than its breadth, which has taken the place of square Roman letters, is probably related to the development of astigmatism of the lens, and thus indirectly a cause of headache. It is asserted on high authority that vertical astigmatism, the commonest form, is not present in the eyes of children before they learn to read. Headache is an exaggeration of the feeling of fatigue. It may be interpreted as the brain’s expression of unwillingness to be made to work; a protest always to be listened to, notwithstanding that it does not necessarily follow that unwillingness to work is the result of overwork. Constipation, irritation of the sensory nerves of the stomach, overdosing of the brain with alcohol, and many other causes, may, through the vaso-motor system, set up the conditions which normally result from activity unduly prolonged. The fact that a central disturbance, headache, results from undue muscular work calls our attention to the double nature of the mechanism concerned in movement. Muscles are set in motion through the intervention of the nervous system. After they have worked to an unusual extent the nerve-centres connected with them grow tired. This, at least, is a legitimate inference from the fact that headache occurs when certain muscles of the eyeball have been subjected to an improper strain. But it must be remembered that the muscles of the eyeball never tire. They do not, like other voluntary muscles, give notice that they are in need of rest. It is not so clear that the central mechanism is in any way involved in the fatigue which is produced by excessive use of arms or legs. The muscles of the limbs (and the central nervous system) are protected by the sensations which originate in muscles when they are overworked. The fact that a weary man can, if a great emergency demands activity, use his muscles with as much vigour as if he were fresh from bed, has been cited as an argument in favour of the view that fatigue is of central origin; but it is an argument which works both ways. A strong emotion causes a fervent response from the nervous system. Tired muscles contract energetically when the impulses which reach them are sufficiently urgent.
Nothing so definitely removes muscle from the category of machines as its liability to fatigue. To speak of a muscle as tired is, of course, to transfer to an object a term which is applicable only to a phenomenon of consciousness; but it is necessary, unless a cumbrous expression is to be used, to designate thus the effect upon the muscle of prolonged activity. The petrol may be low in the tank, but the quantity burnt in the cylinder at each stroke is not reduced. If an isolated muscle is repeatedly stimulated by an electric current of a certain strength, the response which it makes improves for the first two or three induction shocks; then it begins to weaken. At each succeeding spasm the muscle shortens a trifle less than before. More remarkable than the diminution in the amount of work done by a muscle which is growing tired is the prolongation of the time taken both in contracting and in relaxing. Further, it has been shown that the fatigue which accompanies the contraction of an isolated muscle is not a condition dependent upon the shrinking of the store of energy which it possessed when it was first thrown into activity. Muscles undisturbed as to blood-supply, and contracting under the direction of the Will, also exhibit it. Speaking generally, it may be said that the tiring of muscle is not so much due to the exhaustion of its store of food as to accumulation of products of action. Vigour is restored to a tired muscle by passing through its bloodvessels a stream of salt-solution, which brings it no food, but washes away some of its waste. But the problem is far more complex than this. The machinery is not simply clogged with the products of its own activity. If the blood of a tired animal is injected into the vessels of one that is rested, the muscles of the latter exhibit the phenomena of fatigue. Evidently muscle is self-protective. During activity it prepares a “fatigue-substance” which poisons its own nerve-endings, making them worse conductors from nerve to muscle of the commands which descend from the brain. Not only does the fatigue-substance dull the nerve-endings in the particular muscle which has contracted, but, being distributed by the blood to the whole body, it produces a general effect. If the legs have been severely worked, they exhibit fatigue in the highest degree; but after a long walk the arms also are less ready and less capable than the state of their nutrition warrants.
The condition of stiffness experienced for a day or two after excessive exercise is due to various causes in combination. The fact that it may be remedied by encouraging the circulation through the muscles most affected, as by hot baths and massage, tempts us to assign it also in large measure to accumulation of products of action; but the means taken to reduce stiffness favour the nutrition of the muscles both by giving them more food and by carrying off their waste.
Equally remarkable with the self-protective disposition of muscle, which forbids it to give, except at the instance of increasingly urgent messages from the central nervous system, more than a part of the work of which it is capable, is its preparation for meeting an increased demand. It grows with use. Running increases the girth of the leg by developing especially the muscles of the calf. Raising weights enlarges the muscles of the shoulder and arm. Use-growth may reach inconvenient proportions. Nothing is more noticeable during the training of young athletes, whose nutritive responsiveness is at its height, than their liability to pass through a stage in which they are “muscle-bound.” Their legs grow bigger, but their pace falls off.