Although they cannot tell the true significance of the electromotive change which marks the passage of an impulse, physiologists are in a much better position now than formerly to controvert certain popular misconceptions. There is no such thing as “nerve-force” in the vulgar sense. A nerve does not transmit energy to a muscle. The muscle obtains the energy which it dispenses when contracting from the foods with which the blood supplies it. The nerve transmits an excitation. Over-excitability is not a sign of strength, but of weakness. Nor is an impulse in a nerve an electric current. It may be generated by an electric shock, but a chemical stimulus is equally as effective. The slow rate at which it travels, as compared with electricity, puts it altogether out of comparison with an electric current. Its relatively rapid progress, on the other hand, equally excludes the hypothesis that it is a movement of ions, as that phenomenon is observed in solutions of salts.
What is the nature of the process by which energy is conveyed along a nerve? When speaking of the passage of impulses from receptors to the central nervous system, and through this to effectors, we have used the vague expression “molecular change,” to avoid the necessity of being more precise. But the problem is of such profound interest that we look with eagerness for any hint of the direction from which light will eventually be thrown upon it. Recent discoveries regarding the nature of electricity, combined with investigations at present in progress as to the physical constitution of proteid substances, give more than a hint. Hitherto the choice has lain between a chemical and a physical explanation; now the border-line between chemistry and physics, always wavering, has disappeared. The hypothesis that an impulse is a progression of chemical change has meant in the past that the “wave” was due to the oxidation of substances contained in nerve, with liberation of CO₂ and H₂O. Various considerations render such metabolism of the substance of which nerve-fibres are composed improbable. In the first place, nerve-cell bodies contain a store of material, tigroids ([p. 320]), which is recognizably drawn upon during nervous activity. It would appear, therefore, to be the tigroids, and not the substance of the nerve-fibre, which supply the energy transmitted along a nerve. Then, again, the axon of a nerve-fibre, enclosed as it is in a tube of fat, is peculiarly ill-placed for the reception of the nourishment which would be needed to make up for waste, if its metabolism be fluctuating and at times excessive. Nor have nerves more than a very meagre blood-supply. Secondly, observation does not give any support to the hypothesis of fluctuating metabolism. A nerve does not give off more CO₂ when active than when passive. Nor does it become acid. Thirdly, nerves, or, to be quite accurate, medullated nerves, are indefatigable. Their capacity for conduction is not diminished by previous use, as it would be were it dependent upon their reserve of nutriment. These various considerations rule out a “chemical” explanation of the old-fashioned type. It is premature to do more than outline the “physical” theory which seems destined to take its place; and the reader will perhaps forgive if, for the sake of clearness, the case is put with unjustifiable definiteness and simplicity. Proteid substances are constituted of clusters of molecules. The form of the clusters depends upon the salts (or, more precisely, the ions) with which they are associated, and the associations depend upon the electric charges which the ions carry. In resting nerve-protoplasm the clusters are small, and, since the total surface-area of a number of small spheres is greater than the surface-area of the same weight of matter when condensed into large spheres, there is, so to speak, more surface for the ions to cling to. Conversely, when the ions leave the small clusters, the latter are not protected from the influence of mutual attraction. They fuse into larger clusters. Fusion is carried to its extreme limits when a protein coagulates. A nerve-impulse is a “wave” of partial coagulation. The positive electricity generated in a cell-body by the metabolism of its tigroids repels the positively charged ions which cling to the nearest protoplasm-clusters in the axon. Like acrobats swinging from trapeze to trapeze, each flight of ions dispossesses the ions from the clusters in front of it; and in this way the disturbance progresses down the axon as an electric wave.
Thus we interpret the shadow cast by a theory of which either of several pioneers who are diligently climbing may at any time obtain a view. The conductivity of protoplasm (and what is true of its conductivity will be found to hold good equally for its irritability and changeableness of form) is due to the readiness with which its molecules enter into unstable associations with electrolytes. The instability of these associations is related to the tendency of the molecules to cluster. An impulse is passed along a nerve as a displacement of ions; the ions being transferred from one molecule, or group of molecules, to the next. Such an explanation of an impulse involves no chemical breakdown of nerve-substance during its passage along a nerve. It transfers the metabolism which liberates energy (reinforcing the impulses which have originated in sense-organs) to the nerve-cell bodies. It is based upon certain experimental data which appear to have been established; but, like all other hypotheses which are intended to account for physiological phenomena, this one must be brought to the test by varying the conditions under which impulses pass along nerves, and ascertaining whether the consequent alteration in the force, rate, and other attributes of the phenomena are in accordance with physical laws. In applying these tests to the activities of protoplasm, we are, however, met by an insuperable difficulty. The matter which transmits nerve-impulses is alive. We have no laboratory standards by which to judge whether the changes in conduction which are produced by changes in the conditions of the conductor are, or are not, consonant with physical theory. It is with protoplasm that we are dealing, and not with a mixture of proteins in solution. If we surround a nerve with nitrogen, it loses its conductivity in five hours, to recover it when oxygen replaces the neutral gas. This has been regarded as proving that metabolism of the nerve is necessary for the transmission of impulses. But conductivity is a phenomenon of life. Deprivation of oxygen for five hours must bring the nerve-substance to the verge of death. It might be argued that the retention by the nerve for so long a time of its power of conducting impulses shows that its metabolism is not a cause of the phenomenon. Again, it has been shown that warming the nerves of cold-blooded animals greatly increases the rapidity of conduction. It is more than doubled in the nerves of the “foot” of a slug-and a similar increase has been proved for the nerves of a frog-by a rise of temperature of 10° C. Reflecting on the results of this experiment, a physicist would exclaim: “Then an impulse is a wave of chemical change. A rise of 10° C. increases the rate of chemical processes from two to three times; whereas no known physical process is accelerated by more than 5 to 15 per cent.” But the physiologist remembers that a rise of temperature of 10° C. increases all the activities of a frog. He is hardly prepared to say that its greater vivacity may not be the expression of more rapid oxidation; but he sees no fore-ordained balance of vital enterprise and chemical change. He is, or ought to be, extremely suspicious of any explanation which appears to over-ride physical laws; yet, at the same time, he is aware that until he has more accurate knowledge regarding the constitution of protoplasm he will not be in a position to understand how physical laws apply. The protoplasmicity of protoplasm is increased by warmth. What change of molecular constitution does this imply?
The view that in a muscle molecular change gives rise to an electrical change, which in turn produces the change in form, has been very widely held. The hypothesis was based on observations which seemed to show that the electric variation travels a little ahead of the wave of contraction; but every improvement in recording apparatus has diminished this apparent want of synchronism. There can be little doubt but that the lagging behind of the wave of contraction is due to the inertia of the muscle and of the recording apparatus. Molecular change and electric variation are simultaneous. If this be true, the electric change cannot be regarded as the cause of the molecular change, in the sense, at any rate, in which they used to be considered as cause and effect.
The power of muscle varies as its cross-section. For human muscles the maximum lift amounts to from 7 to 10 kilogrammes for each square centimetre. This is a large figure, but it must be remembered that, owing to the arrangement of the bones as levers, most muscles act at a great mechanical disadvantage. The greater the difference in distance from the fulcrum between the point of application of the force and the point of incidence of the weight, when the force acts nearer to the fulcrum than the weight, the greater is the mechanical disadvantage. The greater also is the rapidity with which the weight is lifted. What is lost in strength is gained in swiftness. Contrast the slow steps of a negro, whose long heel separates the point of application of the power (tendo Achillis) from the fulcrum (the ankle-joint), with the springy movements of a European. A European needs, and as a rule has, a better developed calf, which allows him his more sprightly gait, without sacrificing his carrying power. Our preference for slender wrists and ankles is not purely æsthetic, unless we admit, as may be maintained, that all natural canons of taste rest upon utility. Slimness of joints means nimbleness. A few muscles act directly, without loss of power—as, for example, the masseter, which lifts the lower jaw (hence a grand capacity for cracking nuts)—but most muscles move levers of considerable length. Compare with the masseter the biceps and brachialis which lift the forearm. Their tendons are inserted into the radius and the ulna at a distance from the elbow-joint which is about one-tenth as great as the distance from it of a weight held in the hand. Their united cross-section is about 16 square centimetres: (16 × 10) / 10 = 16. One cannot hold out in the hand, the elbow being pressed against the side, so that these muscles alone are acting, a greater weight than 16 kilogrammes (34 pounds), although the muscles are exerting a traction ten times as great as this. The strength of muscle when pulling straight is well illustrated by the thick white mass in the centre of an oyster. It keeps the shell closed until a force equal to 1,300 times the animal’s weight has been applied. This muscle also affords a good illustration of the part played by reflex contraction in opposing stretching—the reaction by which tone is maintained. Anyone who inserts an instrument, such as the end of a screwdriver, between the slightly open valves of an oyster lying under water will find that he needs to give it an exceedingly smart twist if he would catch the muscle asleep. Stretching it causes a reaction proportional to the stretching force.
Fig. 17.—Biceps Muscle in Action.
The fact that the output of energy by muscle is proportional, within certain limits, to the work to be done, is brought out even in laboratory experiments. A nerve-muscle preparation teaches that the amount of work is not a function of the stimulus. Within certain limits a stronger stimulus evokes a higher and stronger lift; but the stimulus remaining the same, the work done by muscle (i.e., the product of weight multiplied by height) is, up to a certain optimum, increased by increasing the weight. Often a very light load is not lifted as high by a nerve-muscle preparation as a slightly heavier one. No satisfactory theory of this reaction to load has yet been formulated. Explanations have been put forward, but they merely substitute one unknown for another, a not uncommon drawback to explanations.
Muscles are strongest when at their full physiological length. As he dips an oar into the water a man exerts the greatest force of which he is capable, provided that he is not guilty of “missing the beginning.” Hands over the stretcher, body between the knees, ankle, knee, hip, fully flexed, arms straight—all his strongest muscles are at their greatest physiological length. Rowing is an exercise which has no rival. Every muscle in the body, from little toe to little finger, comes into play under the conditions which suit it best. And not less admirable is the effect upon the abdominal muscles during recovery at the end of the stroke; and the rhythmic movement which encourages deep and measured respiration.