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.