The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

The development by means of exercises of a strong muscular system has received much attention during recent years. Our ancestors cultivated strength and agility in certain movements without paying much attention to the muscles by which the movements were performed. It is fashionable nowadays to lay stress upon the importance of maintaining an abundant musculature, because of its relation to general “fitness.” The balance between muscular activity and the organic functions which is observed by everyone who takes an active holiday proves beyond doubt that the nutritive condition of the various glands and of the heart and bloodvessels is in some degree dependent upon the condition of the muscles. Possibly they secrete into the blood other “messengers” in addition to fatigue-substance—messengers whose call wakes up the organs of digestion. The man who is so fortunate as to be able to use his muscles in the open air has no need of exercises in his bathroom. Failing out-of-door opportunities, much can be done by the systematic use of the various muscles working against resistance. It is alleged, and we are not disposed to dispute the justice of the contention, that movements made with the fullest degree of mental concurrence have a more rapid effect upon the growth of muscle than actions more or less unconscious. Muscle and nerve are parts of a single mechanism. It may be that fixing the attention on an exercise, and watching its performance in a looking-glass, aids the nutrition of muscles by increasing the influence of their nerves, possibly by improving the nutrition of their nerve-centre. Unfortunately, this is one of many theories which hardly come within the reach of a control experiment. Could one concentrate attention on the movements of the right arm, then absent-mindedly repeat them with equal vigour with the left, it might be possible to ascertain whether there is anything in this idea. Two other contentions with regard to the best way of performing movements, with a view to the promotion of muscular growth, appear to be justified by their results. Working against a moderate or light load is said to be more effective than putting muscles to a severe strain. A small number of maximal contractions, it is said, induce more rapid growth than many partial shortenings. According to this scheme, when a particular muscle needs strengthening, because in a certain action it is to be the chief performer, it is made to bring its two ends as near together as the plan of its attachments allows. Maximal shortening is apparently favourable to blood-supply and otherwise promotes nutrition.

Tone.—Hitherto we have spoken of quiescence and activity, as if muscle were doing nothing when not visibly contracting. A wrong impression may be engendered by these terms. Muscle is never idle. During sleep, and still more when a person is under the influence of anæsthetics, the muscles approach the condition of machines at rest. But again the language of the workshop is inapplicable. When a headless frog is hanging from a hook its legs are slightly bent. All its muscles are weakly contracted, if we understand by contraction a condition in which the length of muscle is less than it would be were it not alive. But the flexors are tenser than the extensors, hence the crooking of hip, knee, and ankle. If the sensory roots of the sciatic nerve are cut, the leg straightens out. So long as the nerve was intact the weight of the limbs acted as a stimulus to sensory nerve-endings, causing a reflex “tone” of the flexor muscles via the spinal cord. The tone of the extensor muscles was less because they were not stretched by the weight of the limbs. Every joint is under the influence of antagonistic muscles which are perpetually watching one another. When the limb is extended the flexors become anxious. When it is flexed the extensors get ready for a spring. Only when it is half flexed is there anything approaching to a truce. And this in most cases is the position of greatest comfort. But even when most at rest, muscles still possess a certain degree of tone. The tendency to shortening in one set causes it to pull against, and thereby increases the tone of, its opponents. When a muscle contracts it does not lift a loose bone. It has to overcome the tone of the muscles which would cause a movement in the opposite direction. And here another adjustment comes into play. The same gross stimulus which leads to the contraction of A starts impulses of a finer kind for B, directing it to relax its tone. We have seen how the heart and bloodvessels are under the influence of two sets of nerves of opposite sign—anabolic, diminishing irritability; katabolic, increasing it. All muscles are under similar management; but we can rarely detect the influence of the anabolic, inhibitory nerves, the brakes, because the katabolic display is overwhelmingly conspicuous. We must be content with two experimental demonstrations. An animal’s hamstrings have been cut; the flexor muscles of its thigh are therefore severed from their attachments below the knee. The tone of the extensor muscle keeps this joint extended. If now the pad of the foot be tickled, the flexor muscles contract, just as they would do if they were still able to carry out the reflex action of raising the foot. They cannot do this, because their tendons are divided; nevertheless, the knee bends owing to reflex relaxation of the extensor muscles. Still more striking evidence of reciprocal contraction and relaxation is afforded by the claw-muscles of a crayfish. A weak stimulus to its nerve causes the claw to set open; a stronger stimulus causes it to close. Both these movements are due, not to “contraction,” but to change of tone. Under certain conditions, a current passed through its abductor muscle, the claw being open at the time, causes closure by inhibiting the tone of this muscle. In this case the stimulus acts directly on the muscle, producing an effect which is opposite to the one we are accustomed to associate with stimulation; in place of contraction, relaxation.

Contraction of muscle, moving something, impresses one as a positive phenomenon. Relaxation seems negative—the undoing of contraction—and to a very large extent this attitude of mind is justified. Return of a muscle to its full length is due either to stretching by the weight it has lifted, or to the antagonism of other muscles. An isolated muscle lying on a pool of quicksilver does not return to its full length after it has contracted. But it is necessary to banish the machine idea. A machine gives out all the energy it has in store. Muscle is extremely parsimonious. No stimulus can induce it to part with more than a fraction of its energy. Recovery is as definite a function as disturbance. A machine starts when a crank is moved, stops when it is replaced. Muscle has a certain degree of automatism, although its tendency to act on its own account has been almost completely transferred to the governing nervous system. Muscle and nerve work together, and the efficiency of muscle depends upon the maintenance of its relations with its nerve. If the nerve is cut, the muscle atrophies. We will not stop to consider whether wasting may be properly attributed to disuse, or to vaso-motor changes. In its lowest form nervous influence shows itself in the regulation of the nutrition of muscle. A somewhat more forcible exhibition of control is seen in the regulation of tone. The maximum is reached when a wave of undoing which has passed down a nerve infects the protoplasm of muscle with the same tendency to disintegration. The muscle-substance explodes. The muscle shortens.

Remarkable evidence of the existence of muscle-tone is afforded by the knee-jerk. Place a person on an upright chair, with his legs crossed, muscles lax, foot hanging free. With a paper-knife or the end of a stethoscope, or even the hand used edgewise, tap the ligament which connects his knee-cap with his shin. The tap is instantly followed by a jerking forward of the foot. The deep muscles of the thigh, vastus, and crureus, have contracted. This phenomenon is easy to account for. When we are standing upright, the trunk is supported on three joints, of which one—the hip—is a perfect ball and socket, and the other two—knee and ankle—are of the same order so far as the absence of any provision for locking them is concerned. If the muscles on the front and the back of the leg did not constantly adjust our balance, by swaying the trunk forward when it falls back, and pulling it back when it sways forward, the joints of the leg would double up beneath us. A photographer knows how little confidence is to be placed in a man’s assertion that he is able to stand still. This see-saw of alternate contraction and relaxation is kept up by means of nerve-impulses which ascend from the nerve-endings surrounding the separate bundles of tendons, or from the Pacinian bodies which are found in abundance in the neighbourhood of tendons and ligaments, or from the elaborately twisted nerve-fibres found in muscle-spindles, or possibly from all three classes. Muscle and tendon are richly supplied with sense-organs susceptible to pressure and stretching. There is an abundance of nerve-endings to choose from. The slightest change in their tension, whether due to the muscle’s own contraction or to the action upon it of other muscles or weights, is recorded not only in the spinal cord, but also in the cortex of the cerebellum, and, if the contraction is an act of volition, in the cortex of the great brain. Although it was skin which was tapped, skin-nerves have nothing to do with the jerk. It was the result of the slight sudden stretching. In short, the tone-mechanism has been fooled. Notice the position of the leg. The knee is semiflexed; the foot is hanging free. There is nothing for the extensor muscles of the thigh to do. Now, if ever, they are justified in dozing. It is not to be wondered at that the sudden stretching of the ligament takes them off their guard, or that on waking they give a quite unreasonable start. The phenomenon is, as we asserted, easy to account for. It would also be easy to explain, if it were not for the extreme rapidity with which the jerk follows the tap. The interval is about one-hundredth of a second. This is thought to be too short to allow an impulse to ascend a sensory nerve, pass through the cord, and descend a motor nerve. It is true that these reflexes of adjustment must stand on a different level to other reflexes. The tone-impulses which cause them are incessantly patrolling to and fro from sense-organs to nerve-endings. The paths they follow must be the most open in the nervous system. Receptors and effectors must, in an electrician’s phrase, be incessantly switched on; or, to express the analogy more accurately, the flexor and extensor tone mechanisms are incessantly and reciprocally switching each other on and off. It must be confessed that it is very difficult to explain the knee-jerk if it be not a reflex action, but, as has been supposed, a direct response of the thigh muscles to their own stretching. The latter hypothesis does not appear to be reconcilable with its dependence upon the maintenance of the nervous connection of the muscles with the spinal cord. It cannot be elicited unless the “spinal arc” is intact. It ceases after the severance of either sensory or motor roots. Nor will it occur if the supply of blood to the lower end of the spinal cord has been cut off. Still more difficult is it to explain its extraordinary sympathy with everything that happens in the whole nervous system, if the impulses which cause it do not pass through the spinal cord. By a very simple mechanical arrangement it is possible to record the amplitude of the knee-jerk. The foot moves a lever which writes on a travelling surface. The jerk is elicited by the hammer of a clock strapped to the shin. In this way it is possible to extend the period of observation over several consecutive hours, the subject becoming completely oblivious of the movement his foot is making once a second, if it be screened from his view. In deep sleep the jerks stop; but the subject may doze, and still jerk follows tap. And the record made by his foot mirrors all the changes in his nervous system. If he clench his fist, the movement is reinforced, as it is when a child cries, a lamp is lighted, his ear itches. There is music in an adjoining room. His foot is the baton which beats fortissimo to Wagner, and is lulled to piano by the “Lieder ohne Wörte.” On a bright day this spinal pulse throbs gaily. It is indolent in dull, depressing weather. The knee-jerk is the physician’s guide to the condition of the nervous system.

Elasticity of Muscles.—Muscles are very extensible, and after stretching return to their original length. Their elasticity is a quality of great practical importance. It enables them to meet sudden resistance without rupture, as when a man alights from a height. At the moment when the feet touch ground elasticity dissipates the shock. The stretching of the muscles then leads reflexly to the increase of their tone. Here we see an advantage in the short reaction-time of the knee-jerk. Tone comes into play long before impulses generated by contact of the sole of the foot with the ground have had time to reach the brain, or even to induce reflex contraction through the spinal cord. The elasticity of the muscles is also of use in the performance of certain sudden actions. A pea is flicked across the room by pressing the thumbnail against the pad of a finger, or a finger against the thumb, and releasing it with a jerk.

An electrical change accompanies an impulse in its passage down a nerve, and a wave of contraction in its passage along a muscle. In 1788 Galvani observed that the hind-limbs of a frog, suspended by a metal hook to metal railings, twitched when the wind blew them against the bars. The hook passed through the lumbar plexus of nerves. He recognized that the cause of the twitch was the closing of a circuit. The birth of dynamic or galvanic electricity dates from this observation; and ever since this phenomenon was first observed the electric changes in nerve-muscle preparations made from frogs’ legs have been favourite subjects of research. Many observations with regard to nerve-conduction and muscle-contraction may be made, and many experiments performed, without special apparatus. A frog having been killed by cutting off its head, or by placing it beneath a tumbler with a wad of cotton-wool soaked in chloroform, the skin of the leg is removed, displaying the khaki-coloured muscles, bluish tendons, and bright white threads of nerve. A stretch of the largest nerve of the back of the thigh, the sciatic, is isolated. All the muscles of the thigh are then cut away and the bone nipped across just above the knee. The bones below the knee are removed, the superficial muscle of the calf, the gastrocnemius, being allowed to hang free, its bifid end attached to the fragment of thigh-bone. Its lower end terminating in the tendo Achillis, with its insertion into the prominence of the heel, is left intact. The bone is fixed in a clamp. A light lever made from a wooden spill is suspended from the tendo Achillis. The nerve may then be stimulated in various ways: by crushing in a pair of forceps, burning with a heated needle, touching with a drop of glycerin or a strong solution of salt. But of all methods of stimulation, the best is the current from an induction coil. Since it does not injure the nerve, it can be applied as often as may be desired. The amateur provided with an induction coil is in a position to study the relation between stimulus and response. He can vary the strength of the stimulus and vary the weight which the muscle has to lift. He can observe the progressive onset of fatigue, and otherwise gain much information regarding the behaviour of muscle as an isolated piece of apparatus.

It is the ambition of the expert to obtain absolutely correct records of the time-phases and of the changes in electric potential of nerve and muscle under varied experimental conditions. For this purpose he needs the finest apparatus which instrument-makers can furnish, and the knowledge and dexterity requisite for its employment. Consider, for example, the record of the change of form. A nerve-muscle preparation, obtained by the method already described, is arranged so that the point of the lever scratches on a rapidly travelling blackened surface. As the muscle contracts it makes a “tracing.” A tuning-fork vibrating at the rate of, say, 400 times a second also scratches a tracing on the same travelling-plate. It is easy to time the several phases of contraction and relaxation by comparing them with the undulations made by the tuning-fork. By means of an induction shock a single impulse is generated in the nerve and a single spasm evoked in the muscle. Our tracing shows that the spasm lasts about one-tenth of a second, and that about half this time is occupied by contraction, and half by relaxation. But the ascending curve is usually a little steeper than the descending curve, and the apex a little nearer to the commencement of ascent than to the termination of descent. An electric signal marked the instant at which the current was sent into the nerve. The time taken by the impulse in travelling from the spot where the electric current entered the nerve to its junction with the muscle can therefore be estimated. The contraction begins so much sooner or later, according as the shock is delivered nearer to, or farther from, the muscle. By shifting the electrodes up and down the nerve, the rate at which the impulse travels is directly measured. After the time that the impulse took in reaching the muscle has been allowed for, there still seems to be an interval before the muscle begins to shorten. This was termed the “latent period,” under the impression that some time is actually lost in turning the nerve-impulse into a muscle impulse. The impulse was supposed to be latent in the end-plates of the nerve. Various hypotheses were formulated as to the nature of the transformation. The progressive improvements in apparatus and methods is testified by the diminution in this latent period as given in the text-books of successive decades. It is now put at ¹/₄₀₀ second, and is regarded by most physiologists as a delay due to the inertia of the muscle. Owing to its elasticity, the molecular change in muscle does not immediately affect its shape. When the latent period appears to be longer—say ¹/₁₀₀ second—the balance is due to the inertia of the recording apparatus. Usually the curve shows a rise lasting ⁴/₁₀₀ second and a fall occupying ⁵/₁₀₀, due to the fact that inertia of muscle and apparatus delays the commencement of the rise, but does not hasten the termination of the fall.

When an impulse is generated artificially by an induction shock, a single spasm or twitch is the result; but in Nature contraction is never limited to a single twitch. Impulses descending from a motor nerve-cell to a muscle are always rhythmic. They follow at the rate of eight or ten a second in human nerves; and since in our muscles contraction and relaxation take longer than in a frog, a second impulse reaches the muscle before the effect of the first has passed away. The muscle has not had time to relax, when it is again called upon to contract. Hence a summation of contractions. The muscle continues to shorten until the maximum of contraction is reached. This condition is termed “tetanus,” to distinguish it from a single spasm. In fullest contraction the length of a muscle may be diminished by one-half, or even by two-thirds.

It would be impossible to treat of the electrical phenomena displayed by nerves and muscles without presupposing some acquaintance with the methods and laws of physics. As this is contrary to our understanding with our readers, we must be content with the statement of a few salient facts. At the moment when an impulse is passing along a nerve, or a wave of contraction along a muscle, the electric potential of the active part of the structure, whether nerve or muscle, is different from that of the not-acting parts on either side of it. A battery in its commonest form is a glass vessel containing sulphuric acid in which a plate of zinc and a plate of copper are immersed. The zinc is electro-positive as regards the copper. In a muscle the contracted portion is electro-positive as regards the parts uncontracted. The degree of positivity can be measured by connecting the muscle at two spots with the two wires of a galvanometer. When one wire makes contact with the contracted portion, and the other with a part which is not contracted, a current passes through the galvanometer, causing its needle to swing; and since the wave of contraction is not stationary, but passes down the muscle, the current is subsequently reversed. The wave, as it were, first tilts up one end, and then, passing on, tilts up the other, letting down the first. The contracted spot is electro-positive to the spot not contracted, and then the latter, contracting, becomes electro-positive to the former, which has relaxed. The needle of the galvanometer swings first to the left, then to the right. The importance of this method of investigation lies in the fact that the electric variation exactly represents, both in time and in intensity, the change which is occurring in nerve and in muscle. By following it, we can ascertain the rate at which an impulse travels down a nerve. We can determine its length and its “form.” Represented on paper, it is a wave. This wave travels in warm-blooded animals with the rapidity of 35 metres in a second. When it reaches a muscle, its rate—that is to say, the rate at which the wave of contraction invades the muscle—is 6 metres in a second. The time during which any particular level in the muscle remains contracted in a single spasm, under the influence of an artificial stimulus, is about 0·05 second. The length of the wave is 300 to 400 millimetres. These measurements give us a very clear idea of the events which occur in a nerve-muscle. An impulse picked up by a motor cell in the spinal cord runs down its axon—termed later a nerve-fibre—with great rapidity. Even the most distant muscle is reached in less than one-thirtieth of a second. From the end-plate of the nerve it travels in both directions along the muscle-fibre—or group of fibres, since each nerve divides into branchets for thirty to forty muscle-fibres—with reduced velocity. Every particle of each fibre rises and falls; but, seeing that the wave of contraction is much longer than the fibre, the whole fibre is in a state of contraction at the same time, although not with equal vigour throughout its whole length.

We cannot dismiss the further consideration of the electric phenomena of nerves and muscles without some inquiry into their meaning. It is evident that they are intimately related to the molecular changes which constitute an impulse. But at present the physics of the phenomena are beyond our grasp. We may speak in a general way of dissociation of ions; but we do not really know what is happening at the spot which is in a state of impulse. We cannot bring the transformation which it is undergoing into line with chemical and physical transformations which we understand. Probably the electrical phenomena which mark it are not peculiar to muscle and nerve. All living changes of state are of the same nature. Cellular activity, or protoplasmic activity, to use a better term, wherever it occurs, is accompanied by electrical change. But it so happens that nerve-substance and muscle-substance have a definite orientation which gives to the electric force a cumulative effect. In a liver-cell it is dispersed in all directions. In a muscle the change of potential at one particle is added to the change at the next, until the sum of all these changes, transmitted along the length of the fibre, is sufficiently large to deflect the needle of a galvanometer. Owing to its summation it attracts our attention.