Chapter X: Muscle (1)

Living matter, protoplasm, is irritable. It responds to influences impressed upon it by its environment. An effective influence, termed a “stimulus,” produces a change in protoplasm at the spot at which it acts. From this spot the change spreads outwards as an “impulse.” Protoplasm is said to “conduct.” A stimulus may be likened to a blow given to a fixed but elastic mass; an impulse to the vibration which travels outwards from the spot struck. Unfortunately, the term “stimulus” is used both for the stick that strikes, the stimulator, or stimulant, and for the blow that is struck; but breaches of logic seldom lead to confusion in an experimental science. The context indicates the particular application of the term. The manifestation of stimulation is a physical or chemical change—most obvious when it is one of form. This change of form may occur at the spot stimulated, or may be deferred to a distant part to which the impulse is conducted.

In opening the study of muscle and nerve we need to form a conception of the nature of these three functions—irritability, conductivity, and changeableness of form. Not that the functions are as distinct as the ideas to which the three terms give rise. They are three aspects of a common function; although this is a reflection which will carry more weight when the ways in which protoplasm reacts to external forces have been considered.

A stimulus may be mechanical, something in the nature of a blow which displaces the particles of protoplasm; or it may be chemical or thermal, disintegrating a portion of its substance; or electrical, divorcing the ions of its molecules. Only the last in any way resembles a natural stimulus; since electrical stimulation alone can be repeated without the substance stimulated showing any evidence of injury in the process. Mechanical, thermal, chemical stimuli destroy a portion of the protoplasm upon which they act. Yet even the weakest of electric currents is a gross disturbance as compared with natural stimuli, such as touch, warmth, sound, light. The essential and most distinguishing quality of living matter is its return to its original state immediately after stimulation. It does not even wait until the stimulator has ceased to act. An effective influence is a sudden change in the environment. It is answered by a sudden response, followed by a return of protoplasm to the state in which it was before the impact of the external force. The change progresses through the protoplasm as a transitory alteration of state, the particles concerned in conducting it returning to their original condition the moment it has passed. No non-living matter responds to force in this way. If a stone is dropped into a pond, a wave circles outwards from the spot it strikes; but this is a wave of displacement, not a change of state. Suppose the pond contained a solution of sugar which the impact of the stone changed into vinegar, and that the zone of vinegar spread outwards, the liquor returning to the condition of sugar and water as it passed. Here we should see some analogy to the progress of an impulse. But no non-living matter behaves like this. A product of the laboratory may be so unstable as to explode when shaken, passing on the slightest provocation into a more stable state. It does not return after the explosion to its previous strained condition. Having thrown away its energy, it continues on a lower plane. Protoplasm parts with energy to recover it again. It returns to instability after assuming a more stable form.

If we are to form a conception of the cause of the irritability of living matter, we must have a mental picture of the physical conditions which distinguish life from death. All matter is in a state of motion. It consists of separate molecules, each moving in its orbit with vast rapidity. A molecule is a cluster of atoms. The dimensions of its orbit depend upon the number and weight of the atoms in its cluster. If we could watch the dance of the molecules of proteins and other substances into which protoplasm breaks up on dying, we should see each separate cluster executing the figure appropriate to its mass, indifferent to the movements of neighbouring groups. But if living protoplasm were of the company, the scene would be one of vastly greater animation; for now it is the ambition of our dancers to form a single group. To this they can never attain. There is a physical limit to the number of dancers who can hold together while the music carries them in wide sweeps backwards and forwards across the floor. At every gust of wind which bursts through an open doorway a group breaks, to clasp hands again as the wind subsides. Protoplasm is always on the verge of instability; always snatching at additional atoms which it draws within its ring; always shaking off other groups of atoms because the ring is too large to hold together. Touch it, and it falls into simpler combinations. Kill it, and it becomes a mixture of organic and inorganic compounds which we know and can name. But as long as it is alive—as long as it is protoplasm, that is to say—integration and disintegration are occurring. Simultaneous complication and simplification _is_ life. The protoplasm-molecule, if we dare to think of it as a molecule, in the sense in which a chemist uses the term, is always changing. It is its variability which makes stimulation possible. Irritability is a tendency to dissociation under the influence of an external force, with reassociation when the force ceases to act.

The molecules which protoplasm gathers into itself may be classified under the headings oxygen, foods, water, and inorganic salts. It is the two latter which most affect its state, conferring upon it the capacity of exhibiting the phenomena of life. Water and the ions of salts dissolved in water, electrolytes, are linked to the other elements of its groups. Striving to find room for more molecules of water and more ions, protoplasm expands. It becomes more mobile and more irritable; for irritability and mobility vary as the number of these extraneous groups of atoms which protoplasm is in a position to let drop. As an impulse travels through it they lose their hold, recovering it as they pass the impulse on. This progress towards expansion is the lifeward tendency; the quickening of activity which leads also to the incorporation of additional atoms of nitrogen-containing substances, and consequent growth.

The opposite tendency is deathwards. Protoplasm drops extraneous groups of atoms; retires into itself; loses irritability; settles down to rest.

The molecules of proteins exhibit a property which appears to pertain in some degree to living matter also. When their relation to the water in which they are dissolved and the electrolytes which it contains is disturbed, they appear to go out of solution, they coagulate. This disturbance is brought about in all proteins by heat; in some it is the result of altering the amount of salt in the water in which they are dissolved. Coagulation in protoplasm is the prelude to death; but it would appear that a step in this downward path is taken whenever an impulse is conducted. Coagulation is due to the clustering of the molecules of a protein. When protoplasm drops electrolytes and water its molecules cluster in some degree, regaining their independence and reattaching their accessory groups of atoms as the cause which drove them to make for safety passes by.

Our conception—not of life, but of “the physical basis of life,” may be very wide of the mark. The account given above is intended as little more than a hint of the lines along which thought is travelling at the present time. The reader must not regard it as a serious attempt to present in detail the views of any of the workers who are endeavouring to apply the results of recent discoveries in molecular physics to the solution of problems in the chemistry of living matter. There can, however, be little doubt but that we are on the eve of further advances which will secure data upon which it will be legitimate to construct hypotheses. At present it would be unreasonable to do more than indicate the direction from which it may be hoped that light will shine.

A stimulus is a change of circumstance rather than a transient disturbance. When an electric current is thrown into it, protoplasm dissociates—parts with something. It instantly reassociates. The continued passage of the electric current does not maintain it in a dissociated condition. When the current is cut off, the sudden change again acts as a stimulus. Within limits, the efficiency of an electric stimulus varies as its suddenness. Similarly with all other stimuli to which protoplasm responds: crushing, burning, chemical decomposition, are effective at the moment of their occurrence. When they generate a succession of responses, it is because they continue to produce changes in the protoplasm. Their continued action does not, under ordinary circumstances, prolong the response.

Response to stimulation travels as an _impulse_ through protoplasm. An impulse is commonly likened to a wave, but enough has been said already to prove that the simile is misleading. It is not of the same nature as the wave which a stone starts on the surface of a pond, a pulsation of sound through air or water, an undulation of light or heat in the æther. These various kinds of waves are waves of displacement, a swing first to one side and then to the other. An impulse traverses protoplasm, whether it be the apparently diffuse protoplasm of a leucocyte or the severely oriented protoplasm of a nerve or muscle, as a change which may be described as chemical, with reservations as to the meaning allowed to this term. We may without impropriety represent the fall (dissociation) and subsequent rise (association) graphically as a wave; but even then it is but a half-wave, and inverted. It is a very different thing to the onward progression of an accession of force, with which it is not infrequently confused.

All protoplasm is not equally susceptible of stimulation. Probably it is safer to put this in a different form. Protoplasm is not everywhere equally exposed to stimulation, nor is it when especially exposed to stimulation in one way equally accessible to all other effective forces. A sense-organ is a collection of cells in which protoplasm is so disposed as to be susceptible to a certain kind of stimulus. It is a “receptor” for a particular force. At the same time it is essential to its efficiency that it should be insusceptible to other forces. The protoplasm in certain of the sense-organs of the skin dissociates when compressed, in others when warmed. The cells of these receptors have a certain structure which exposes their protoplasm in such a manner that it cannot escape dissociation when, in the one case, the cells are squeezed, or when, in the other case, they are heated. The ear contains sensory cells so constructed that the protoplasm which they contain dissociates when affected by pulsations of sound. In the receptors of the tongue and the nose protoplasm is exposed to the influence of chemical stimuli; in the eye it is exposed to the dissociating action of light.

Protoplasm is responsive to external force. It conducts the impulses to which stimulation gives rise. Eventually the impulses, which travel along strands of tissue highly specialized for the purpose of conduction—nerves—reach collections of protoplasm which are so disposed that when they dissociate energy is set free. A comprehensive term is much needed for the connotation of this third essential property of protoplasm, the capacity of liberating energy which characterizes “effectors.” An external force, so small in intensity as to be negligible when we are dealing with the body’s accounts, acts upon the protoplasm of a receptor. A change in state results. The change is conducted to an energy-liberating organ. This organ is supplied with blood which brings it food. Food is its store of energy, the raw material from which it manufactures its ammunition. When an impulse reaches an energy-liberating organ its protoplasm dissociates. But here the protoplasm is so disposed—the cells which contain it have such a form—that when it dissociates a change in the cell follows; it alters in shape, or it discharges into its environment heat, or electricity, or light. The dissociation and reassociation of the protoplasm of an effector involves chemical change. Molecules of water and of carbonic acid are cast off. The energy sacrificed in letting matter fall into these very stable forms is the energy made visible, as it were, in lifting a weight or dispersing heat. It must be replaced if the organ is to retain its power of acting when next an impulse reaches it. To replace it, protoplasm takes up food and oxygen from the blood.

The liberation of energy which occurs when a muscle contracts is not a special phenomenon—something which does not occur when the muscle is at rest. It is an intensification of a process which is always taking place. The substance of muscle, like that of nerve and every other tissue, is always combining with oxygen and giving off water and carbonic acid. When we are auditing the body’s accounts, we enter so and so much food and oxygen on the debit side, we credit it with the same weight of water and carbonic acid; or we debit it with the energy potential in the food, and enter to its credit the mechanical work done and the heat set free by the oxidation of this food. Food is the petrol the combustion of which causes the movement of the car. The external force which stimulates a receptor is too insignificant in amount to be carried to account. Physiologists neglect it, just as engineers neglect the energy liberated by the sparking-plug which ignites the petrol, when they are estimating the efficiency of a motor.

Compared with the amount of energy actually received from the environment when a sensory cell of the eye or ear is excited, the energy needed to start an artificial impulse in a nerve is relatively enormous; yet a well-known comparison of the energy conveyed to the nerve in a certain experiment with a nerve-muscle preparation from a frog, and the energy expended by the muscle in contracting, brings home to our minds the fact that it is impossible to carry even this item to account. The energy furnished to the nerve from an electric condenser measured 0·001 erg; the energy expended by the muscle reached 100,000 ergs.

It is easy to determine the amount of mechanical work which results from a given expenditure of energy. By alternately flexing and extending the joints of his legs, a man lifts his own weight up a hill of a certain height. The work can be measured in foot-pounds or in kilogrammetres. But this by no means accounts for all the energy potential in his food. A still larger amount is expended for the purpose of keeping the body warm, or, not improbably, making it too warm; in either case generating heat which is dissipated into the atmosphere. When a machine is being planned, attention is concentrated upon the problem of how to get the largest result in work for a given quantity of fuel. Fuel costs money. All energy dissipated as heat is wasted. Every ounce saved makes for economy. Engineers therefore speak of the “efficiency” of an engine as the relation between the work actually done and the work which would have been done if no energy had been wasted. In the best steam-engines it stands at about 1 to 10. Since the chief function of muscle is to do mechanical work, physiologists are apt to adopt the engineer’s point of view. But in the case of muscle this is justifiable only in a limited degree. The body of a warm-blooded animal is maintained at a temperature higher than that of the surrounding air. Muscles are the chief producers of heat. If they turned all the energy which they receive into work, they would be inefficient as regards this very important function. Yet even from the engineer’s point of view muscles are more efficient than the best of engines.

It is almost impossible to determine with accuracy, in regard to isolated muscles, the amount of food taken up from the blood, and the return in work by the muscles of the energy potential in the food. Calculations have to be based upon observations of food consumed, gain or loss of body-weight, work done by a man or an animal during a period lasting for several days. We shall consider the evidence obtained in this way in a subsequent section (p. 149). But whether we study isolated muscles or the body as a whole, the relation between work and heat varies within wide limits. So wide, indeed, are the variations as to justify the conclusion that there is no necessary relation between the two phenomena. Muscles develop heat when they are quiescent. Activity is accompanied with an increased evolution of heat; but, if it be desirable, the evolution of heat is reduced until it is, relatively to the output of work, much smaller than in the case of any engine which has yet been made. It is sufficient in this connection to state that, under certain conditions, the return in work may amount to about one-half. The comparison with an artificial motor, of whatever kind, breaks down. In an engine combustion develops heat, heat causes steam or gas to expand, the expanding gas pushes a piston. In muscle certain of the carbon, hydrogen, and oxygen atoms contained in protoplasm combine to form water and carbonic acid—compounds too stable to be reassociated with the remaining atoms of the protoplasm-molecule. They are replaced by complex, energy-yielding substances—foods—and by oxygen, carried in the blood. Their displacement brings about a change in the form of the molecules which involves, owing to their peculiar orientation, a change in shape of the muscle as a whole. Such an explanation is, perhaps, more exact than our knowledge at present warrants; or rather let us say, since we do not know what the expression “the form of a molecule” means, it has an appearance of an exactitude which does not characterize it. It is merely intended to help the reader to realize the hopelessness of attempting to compare muscle with any mechanical contrivance. In the boiler of a steam-engine heat is applied to water until its molecules cannot remain in so close a state of aggregation. Their orbits are greatly increased. The cause of the thrust given to the piston of an engine is the increased amplitude of movement of the molecules of steam behind it. In a combustion engine a mixture of petrol and air is ignited. Energy is set free by the resolution of unstable petrol into stable water and carbonic acid. This energy heats the gases, causing them to expand. Waste of energy as heat is inevitable in a machine which depends for its motive-power upon the translation of molecules. The source of muscular force (if it be not intramolecular change) is certainly not, directly, increased amplitude of molecular swing.

But we must not conclude, as we are tempted to do, that muscle is capable of liberating as mechanical work the whole of the energy supplied to it in food, seeing that its activity is always accompanied by evolution of more heat than can be attributed to friction. If the bulb of a thermometer be inserted into a group of muscles, the instrument shows a marked rise of temperature when the muscles contract. Even though the temperature of the chamber in which an animal is placed is equal to its own, the animal makes more heat if compelled to work, notwithstanding the fact that the consequent rise of its body temperature may prove fatal.

Nor can muscles dispense unlimited heat without doing mechanical work. If I am too cold, the obvious means of getting warm is jumping about. There appears to be a level of heat-production which cannot be exceeded without movement. When more heat is called for than quiescent muscles can produce, they exhibit flickering contractions, shivering, without moving the limbs. The signal for increased production is given by the skin. The skin is sensible of the amount of heat which is being lost. Exposure to cold air makes one shiver, by suddenly withdrawing heat. But an increase of temperature in the blood behind the skin has an exactly similar effect. In the first stage of fever, when the temperature of the body has risen two or three degrees, and before the system has become accustomed to this state of affairs, the skin announces to the muscles that heat is being rapidly lost. A severe shiver, termed a “rigor,” is the result. At the same time loss of heat by evaporation is checked, just as it is when the skin is cold. The sweat-glands are rendered inactive. A phenomenon which marks the nightly fall of temperature in consumptive patients is the sudden return of activity in these glands.

Muscle when most highly developed has an extraordinarily definite structure. It is minutely subdivided into units which appear, looked at separately, simple in design. We are tempted to believe that the explanation of the way in which each of these units works is not far to seek. It is disappointing to be obliged to admit that, notwithstanding all the thought which has been devoted to the problem, we are as far as ever from a definitive solution. We understand the principles on which steam-engines, combustion-engines, electric motors are planned. We compare muscle with each of these mechanical contrivances in turn, expecting to discover the principle of its construction. Many ingenious hypotheses have been formulated; but the fact that some of these are mutually destructive shows clearly enough that as yet no approach to certainty has been made. Probably the fundamental error lies in attempting to compare muscle with a mechanical contrivance. The apparent simplicity and regularity of structure of “striped muscle” misleads us. We ought to have commenced our investigations at the other end of the scale of mobile tissue—to have begun with semifluid and apparently homogeneous animal matter, working upwards to the tissue which, being limited to the one function of movement, and movement in one direction only, has, as it were, crystallized along the lines of force.

All protoplasm is mobile. Its particles move one on another. Hence follows either circulation of the living matter within the cell or change in shape of the cell. The two phenomena are identical in nature. Circulation is best studied in a large-celled, transparent part of a plant. A filamentous water-weed is suitable for the purpose. If this be examined with a microscope while still alive, its cells are seen to contain a watery juice enclosed in spaces of denser cell-substance. Bridges of cell-substance span the spaces. The particles of which these bridges consist are in a state of constant streaming motion, which has, it is needless to say, no effect upon the shape of the cell (_cf._ p. 9).

The unicellular animal amœba, leucocytes, and certain spores of plants, are devoid of cell-wall (_cf._ p. 28). Their soft protoplasm is not limited by a rigid case. When it streams, the form of the cell is changed. True, we must not think of the body-substance of an amœba as homogeneous. It exhibits an internal structure. Yet its architecture is not, so far as we can see, sufficiently fixed to restrict the directions in which it can stream. Any change of shape is possible. We cannot find in Nature an isolated clump of living protoplasm; nor do we suppose that, if we found it, it would prove to be homogeneous. It appears to be necessary that protoplasm and metaplasm—the terms have no chemical significance; “primary” and “secondary,” or “chief” and “subsidiary” would be equally distinctive—should be intermixed. Streaming is apparently due to alterations in the surface relations of the two substances.

In multicellular animals certain elongated cells are arranged in groups, with their long axes all pointing in the same direction. They can change in shape, diminishing in length, with equivalent increase in breadth. Since all the cells of a group undergo this change of form at the same time, the result is an alteration in the shape of the animal of which they are a part. Applying the experience which we have gained in studying the movements of unicellular organisms, we conclude that these elongated cells are composed of two substances—protoplasm and metaplasm. The restriction of their capacity for altering their shape to one direction indicates that their protoplasms and metaplasm are not indifferently mixed. The two substances set in lines in the direction of the long axis of the cell. Hence, when streaming occurs—when the force which keeps the molecules of protoplasm and of metaplasm in their respective rows is relaxed—the lines thicken. The cell broadens, with an equivalent diminution of length.

Muscle-fibres exhibit all degrees of specialization. The simplest, “plain muscle-fibres,” are found in the wall of the alimentary canal, of bloodvessels, of ducts, in the tissue of the spleen, in the skin, and elsewhere. Each fibre is a fusiform cell. Save for its central nucleus and a little granular protoplasm in which the nucleus is embedded, the cell may show no architectural features. But in most varieties of plain muscle, and especially in that of the alimentary canal, the substance of the fibres is striated longitudinally. This is visible evidence of the orientation of the molecules of protoplasm and metaplasm in the direction of the long axis of the fibre. It shows that the streaming of particles occurs along these lines. It is, as it were, a diagram of the lines of force.

Heart-muscle has been described already (p. 224). Its striation, which is both transverse and longitudinal, is so delicate as almost to defy microscopical analysis. The transverse striæ are the darker and more distinct. But close examination shows that the transverse striæ do not indicate the direction in which the particles of cell-substance are oriented. They are oriented longitudinally. The cell is a bundle of rods of substance A, embedded in substance B. The transverse markings are very thin lines which cross the bundles at right angles.

The third variety of muscle is the kind by which locomotion is effected. It is present in large masses—all the red tissue to which the term “meat” is commonly applied. It accounts for about 35 per cent. of the body-weight. This kind of muscle is not composed of single cells, but of compound cells, or cell-complexes, termed “fibres.” A fibre may attain a length of upwards of 2 inches, with a breadth of about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to a bone, by the other to a tendon; and since they are shorter than the muscle as a whole, the tendon commences as a membrane which covers the surface of the muscle, sloping to it from the bone to which by their other ends the fibres are attached. A fibre is developed from a single cell. The cell elongates, its nucleus divides, and the daughter-nuclei divide until several hundred have been formed; but cell division does not follow. The result is a cylindrical mass enclosed within a delicate membranous sheath, the sarcolemma. In the early stages of its development its nuclei are in the axis of the fibre, but subsequently they are displaced outwards. In the most highly specialized muscle, known as the “white” variety, they lie just beneath the sarcolemma (_cf._ Fig. 16, B).

The feature of this type of muscle is its transverse striation, almost mathematically regular. Commonly striated muscle is spoken of as “voluntary,” because, for the most part, it is under the control of the Will; but the term, in so far as it implies a connection between structure and mode of actuation, is misleading. Transverse striation is evidence of capacity for rapid action. The muscles which the Will directs exhibit promptitude; but striated muscle, which is not under the direction of the Will, is found in certain situations—_e.g._, the upper part of the œsophagus. Conversely, many animals can voluntarily call into action muscle which is not striped. A turkey erects its feathers by setting in motion little groups of “plain” fibres, which pull on elastic tendons attached to the tips of the buried ends of their shafts. Plain muscle contracts less promptly and relaxes more slowly than the striped variety. Cardiac muscle is quicker in acting than plain, but does not hold the contraction so long.

All striped muscle is not equally rapid. Two varieties are distinguishable: “white fibres,” which respond suddenly to a single stimulus and quickly relax; “red fibres,” which respond in a more leisurely way, but remain contracted longer. In some muscles these two types of fibre are intermixed. Others are wholly red or wholly white. Everyone is familiar with the contrast which the white flesh of a turkey or of the domestic fowl presents to the red flesh of game-birds and birds of prey. In the breast of a blackcock a sheet of white muscle overlies a mass of red. When the bird is cooked the difference in colour is strongly marked. Of the two muscles which, in a rabbit, correspond to our muscles of the calf, the superficial, gastrocnemius, is white; the deeper, soleus, red. The former acts over both knee and ankle joints; the latter over the ankle only. The muscle which, acting over a longer range, has to contract more quickly is white; the shorter, more slowly acting muscle is red. Experiment shows that red and white muscles are distinguished by a difference in the promptitude with which they respond to an electric current. It shows, too, that the white muscle is exhausted sooner than the red. It cannot give so many successive responses to stimulation without a rest. We shall find, when we are considering the minute structure of striped muscle, a difference between its two varieties which we can correlate with their different modes of action. All human muscles belong to the red kind.

The most efficient muscle-fibres in the animal kingdom are found in insects. This will not surprise anyone who thinks of an insect’s power of movement. If a man could jump as many times his own height as a flea can, he would clear the dome of St. Paul’s. An ant can drag an object sixty times as heavy as itself, with no wheels beneath it to diminish friction. Under the same conditions a horse cannot drag much more than its own weight. A dragon-fly, it is asserted—although we have not met a man who guarantees that he has made the observation—will support its heavy body in the air by the rapid vibration of its wings for four-and-twenty hours without alighting. The chirp of a cricket is produced by the rubbing together of its hind-legs. A mosquito sounds its war-cry much in the same way. The pitch of the note proves that the insect’s muscles are contracting and relaxing at least 300 times a second. None of these figures must be applied without qualifications in estimating the relative strength of insect and human muscle. Weight for weight, the muscle of a flea is not so much stronger than ours as the figures might lead one to infer. To ascertain the numerical relation, it is necessary to compare the total cross-section of the two chief segments of a flea’s leg with the cross-section of the extensor muscles of a man’s thigh and calf, and a man’s weight with the weight of a flea. Nevertheless, after all deductions have been made, a considerable balance of superiority lies with the insect as regards the strength of its muscles, their rapidity of contraction, and power of repeating contraction without fatigue. An insect’s muscle is the most suitable that can be obtained for microscopic examination. Its pattern is larger and more distinct than that of other animals. That the pattern should be larger is not quite what might have been expected. It would not have surprised us had we found the pattern finer in the more effective type.

Nothing is easier than to mount a specimen of insect-muscle. The large water-beetle (_Dytiscus marginalis_) is an excellent subject. It is so easily handled. Having cut off the animal’s head, a leg is pulled out from the thorax. It is split open with a penknife, and a little of the muscle is dug out from within its hard case, placed on a clean slide, and covered with a cover-slip. If the preparation has been made quickly and cleanly, the muscle remains alive for five or ten minutes. Not only can it be studied, with the microscope, unaltered by reagents, but under the most favourable circumstances the progress along its fibres of waves of contraction can be watched. The structure of the fibres is more easily made out if a little salt-solution or white of egg is added to the preparation.

A, The nuclei are in the core of the fibre. B, The
nuclei lie immediately beneath the sarcolemma. The
disc on the left of this fibre, and the fibril on
its right, show the two ways in which striated
muscle-fibres tend to cleave. The dark line, or
row of dots, is known as Dobie’s line, or Krause’s
membrane. The figures are severely diagrammatic.]

Striped muscle is crossed by bands, dim, bright, and dark. The sequence is as follows: Starting with the very thin dark line, which often appears as a row of dots, the next band is bright; then comes a dim band about twice as broad as the bright one; then another bright band. This sequence is repeated with extreme regularity from end to end of the fibre. Usually the bands cross the whole breadth of the fibre, although occasionally it is divided by longitudinal lines into parts in which the stratification is shifted a little backwards or forwards. A segment of a fibre comprises the substance between two dark lines—_i.e._, two bright bands with a dim one between them. If the muscle has been hardened in one of the fluids commonly used for the purpose of preparing tissues for the microscope, with its two ends fixed, say, by binding them to a piece of a match, so that it could not shrink, a thin clear line appears crossing the middle of the dim band. This seems to show that the fibre is not made up of single dim discs between two bright discs, but of couples, comprising half a dim disc and a bright disc. The thin dark lines indicate that the fibre is divided into compartments by transverse septa, which are probably reticulated. The appearance of a transverse line of dots, in place of a continuous line, is due to the existence of very fine longitudinal markings (it is unsafe to give them a name which connotes structure). Where the longitudinal lines cross the transverse lines, the optical effect is the appearance of a dot.

If pieces of muscle are placed in a solution of osmic acid, they become hard and brittle, and their markings are accentuated. Muscle from the claw of a crab or a lobster is very suitable for this purpose, owing to its exceptional freedom from connective tissue. After this hardening the fibres are easily separated with the aid of needles into fibrils immeasurably slender. An isolated fibril shows with extreme distinctness the alternation of dark, bright, dim, bright, dark markings already described. The appearance of a cross-section of a fibre also proves that it is a bundle of fibrils. The cut ends of the fibrils appear as dots surrounded by homogeneous substance. In this respect there is an important difference between red muscle and white. In the red fibres the fibrils are fewer and thicker than they are in white, and the embedding substance is more abundant. It is generally assumed that the homogeneous substance, sarcoplasm, is the nutrient protoplasm of the fibre, the fibrils the contractile elements. The more complete the differentiation of the fibre into fibrils, the more rapid is its action; the more abundant the sarcoplasm, the greater its capacity for continued work.

If a living muscle-fibre is observed while a wave of contraction is passing down it, the ends of the fibre being free, so that its shortening is not prevented, it is noticed that the widening of the fibre is accompanied by the thinning, even to obliteration, of the bright bands. The dim discs extend laterally, without any noticeable diminution of their thickness. It looks as if the bright discs, or something contained in the bright discs, were absorbed into the dim discs. The fibre is, as we have already pointed out, striated longitudinally. The striation is more clearly visible in the dim discs than it is in the bright ones. That the dim disc has an architectural structure absent from the bright disc is placed beyond doubt when a muscle-fibre is illuminated with polarized light. The dim disc is then found to be doubly refracting; the bright disc is not. When the prism in the tube of the microscope is placed with its axis at right angles to the axis of the prism which intervenes between the source of light and the stage of the microscope, a succession of bright bands is seen corresponding to the dim bands seen with unpolarized light. The rest of the fibre is invisible, because it has not the property of twisting the undulations of light which the lower prism has set all in the same plane. Various hypotheses as to the cause of contraction, or, to speak more correctly, as to what happens during contraction, have been based upon the thinning of the bright discs. It is assumed that the dim discs have a definiteness of structure which the bright discs do not possess. They are thought of as being traversed by pores, or as consisting of short rods. Microscopists who take the latter view believe that during contraction the more fluid substance, sarcoplasm, which occupies the bright bands is drawn into the dim bands between the short rods, or sarcostyles, which are consequently separated more widely.

No tissue could be more unsuitable than muscle for microscopic examination; for none other offers the same optical difficulties. This will be evident to anyone who considers the description already given of the markings which it exhibits. Whatever may be the true interpretation of these markings, it is clear that they point to an almost infinite multiplication of minute elements adjusted with absolute accuracy side by side and end to end. A cylinder filled with these transparent objects has to be viewed by transmitted light. The elements, whatever may be their nature, refract light in different degrees. It is impossible to eliminate the effects of internal reflection, refraction, and interference of waves of light. The most alluring hypothesis must be accepted with a considerable amount of reserve. Any fact which seems to militate against it must be taken into consideration. The view set forth above, in general terms, is very attractive to everyone who wishes to bring muscle within the category of machines. Suppose we accept the hypothesis that the dim band is a plate made of sarcostyles surrounded by sarcoplasm then the impulse which reaches a fibre causes an alteration in the surface relations of the rods to the substance in which they are embedded. Molecules of fluid from the bright bands are drawn in amongst them; the rods are pushed farther apart; the fibre broadens with a corresponding diminution in length. This brings muscular contraction into the category of the phenomena which play the most important rôle in bringing about the varied activities of the animal mechanism. Contraction is due to osmosis.

The separation of muscle into fibrils after hardening does not seem to bear out either the rod or the pore hypothesis of the structure of the dim disc. It must be remembered, however, that before the fibrils are teased apart the substance of the fibre has been coagulated. The fluid in the bright disc may thus have become as much a part of the fibril as the rod in the dim disc. The longitudinal striation of plain muscle and the appearance of continuous fibrillation in heart-muscle is more difficult to reconcile with the hypothesis that striped muscle is composed of interrupted rods.

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.