Physiological Riddles.

I.-HOW WE ACT.

WHEN a common reader takes up a physiological work, his feelings are apt to be those of admiration, rising rapidly to astonishment, and soon sinking into despair. The multiplicity of the facts, the ingenuity of the experiments, the intricacy of the results, the astonishing amount of light, and the insuperable darkness,-produce a mingled effect upon the mind. The more observations multiply, the more doubtful everything becomes. Thus some recent books assure us that we do not know why we feel hungry, nor what takes place in respiration, nor why the blood circulates, nor why we are warm. Surely these are rather negative results of a positive philosophy. And the worst is, that so much questioning of the past almost shakes our confidence in the present. Do we really know anything on the subject? Shall we find out by-and-by that we do not live by the oxygen of the air, that the blood does not circulate, that food is a fashion, and animal heat an agreeable fiction for a cold day? Is there anything certain in physiology at all, besides what we can see?

If there is, it must be by virtue of some fixed principles; some certain and unquestionable relations established between things. And these indeed scem to be sadly wanting in this department. We appear to be, in physiological inquiries, entirely at the mercy of our senses. Anything might be true, nor can we grasp any fact with a firmer hold than mere empirical inquiry can afford. Every inference, therefore, is open to doubt; no law is ascertained which can sustain the shock of apparent exceptions, nor any principle established to which we may with confidence seck to reduce anomalies. No science has made real progress till it has passed out of this state. So long as no certain principles or necessary laws have been discovered in any branch of knowledge, we cannot tell what we may believe, and, at the best, its doctrines form a mass of truth and error inextricably mixed.

If, therefore, any relations in the vital processes could be ascertained, which must in the nature of things be true, like the propositions of geometry, or if any physiological laws could be found, based on a sufficiently wide induction to give them authority as standards, like the laws of gravitation in astronomy, or of definite proportions in chemistry, this would be a great aid both to the comprehension and to the advance of the science. And though we do not intend here to enter on any such inquiry, we will try whether a clearer light cannot be thrown upon some of the points on which the main interest of physiology centres.

Too much must not be attempted at once. So, dismissing for the present all other subjects connected with the living body, we concentrate our attention on the question, Whence comes its active power? Taking

the body as it stands, supposing it originated, developed, and nourished, by means which we do not now consider, we ask ourselves, Can we find the reason of its spontaneous activity ?—why action should go on within it, and force be exerted by it on the world around?

There is a term we shall find it convenient to use in this inquiry, and may, therefore, briefly define. The actions of a living body are called its "functions." One of these functions is muscular motion, whether external or internal; another is the nervous action; and a third includes various processes of secretion. The growth and nourishment of the body we do not include among the "functions," as we propose to use the term.

We inquire, then, why the living body has in itself a power of acting, and is not like the inert masses of merely inorganic matter? And here let us first observe, that some other things besides the animal body possess an active power. "It died last night," exclaimed the Chinaman, in triumph, on selling the first watch he had ever seen. And certainly a watch is like an animal in some respects. Under certain conditions, it has an active power as like that of the heart as could readily be devised. What are those conditions? They are very simple. It must contain a spring in a state of tension: that is, force must have been applied to it in such a way as to store up power, by opposing the tendency of the metal to straighten itself. Let us fix in our minds this conception of a tension, or balancing of two forces in the watch-spring. The power applied in winding it up is exerted in opposing the elasticity of the steel: it is compressed-coerced. The production of motion from it, when in this state, is a quite simple mechanical problem: let it unbend, and let wheels and levers be at hand to convey the force where it may be desired.

Let it be observed that the force thus exerted by the spring, and on which the "functions" of the watch depend, is truly the force that is applied by the hand in winding it up. That force is retained by the spring, as it were in a latent state, until it is applied to use: it exists in the spring as tension—a state intermediate between the motion of the hand in bending it, and of the hands of the watch in their revolutions. But the motion is the same throughout. It is interrupted and stored up in the spring; it is not altered. We may say, that the tense spring is the unbent spring plus motion. It embodies the force we have exerted. It is not the same thing as it was in its relaxed state; it is more. And it can only pass again into the unbent state by giving out the force which has been thus put into it. Steam is an instance of a similar thing. Water, in passing into vapour, absorbs or embodies no less than 960 degrees of heat. Vapour is not the same thing as water; it is more-it is water plus heat. Nor can it return into the state of water again, without giving out all this heat. Vapour, therefore, in respect to force, is like a bent spring, and water is like the spring relaxed.

And further, as a bent spring tends constantly to relax, and will relax as soon as it is permitted, or as soon as ever the force which keeps it bent is taken away, so does vapour constantly tend to return to the state of

water. It seeks every opportunity, we might say, of doing so, and of giving out its force. Like the spring, it is endowed with a power of acting. Let but the temperature of the air be cooled, let a little electricity be abstracted from the atmosphere, and the force-laden vapour relaxes into water, and descends in grateful showers.

In the vapour, heat opposes the force of cohesion. It is not hard to recognize a tension here; the heat being stored up in the vapour, not destroyed or lost, but only latent. And when the rain descends, all this heat is given off again, though perhaps not as heat. It may be changed in form, and appear as electricity for example, but it is the same force as the heat which changed the water into vapour at the first. Only its form is changed, or can be changed.

Now the living body is like vapour in this respect, that it embodies force. It has grown, directly or indirectly, by the light and heat of the sun, or other forces, and consists not of the material elements alone, but of these elements plus force. Like the vapour, too, or like the spring, it constantly tends to give off this force, and to relax into the inorganic form. It is continually decaying; some portion or other is at every moment decomposing, and approaching the inorganic state. And this it cannot do without producing some effect, the force it gives off must operate. What should this force do then? what should be its effects? What but the

"functions?"

For the force stored up in the body, like all force, may exist in various forms. Motion, as the rudest nations know, produces heat, and heat continually produces motion. There is a ceaseless round of force-mutation throughout nature, each one generating, or changing into, the other. So the force which enters the plant as heat, or light, &c., and is stored up in its tissues, making them "organic"*—this force, transferred from the plant to the animal in digestion, is given out by its muscles in their decomposition, and produces motion: or by its nerves, and constitutes the nervous force.

In this there is nothing that is not according to known laws. The animal body, so far, answers exactly to a machine such as we ourselves construct. In various mechanical structures, adapted to work in certain ways, we accumulate, or store up, force: we render vapour tense in the steam-engine, we raise weights in the clock, we compress the atmosphere in the air-gun; and having done this, we know that there is a source of power within them from which the desired actions will ensue. The principle is the same in the animal functions: the source of power in the body is the storing up of force.

But in what way is force stored up in the body? It is stored up by resistance to chemical affinity. It is a common observation, that life seems to suspend or alter the chemical laws and ordinary properties of bodies; and in one sense this is true, though false in another. Life does not

*As heat, we may say, makes water "gascous."

suspend the chemical or any other laws; they are operative still, and evidence of their action is everywhere to be met with; but in living structures force is employed in opposing chemical affinity, so that the chemical changes which go on in them take place under peculiar conditions, and manifest, accordingly, peculiar characteristics. If I lift a heavy body, I employ my muscular force in opposing gravity, but the law of gravity is neither suspended nor altered thereby; or if I compress an elastic body, my force opposes elasticity, but the laws of elasticity are not thereby altered. In truth, the forces of gravity and elasticity thus receive scope to operate, and display their laws. Just so it is in the living body. The force of chemical affinity is opposed, and thereby has scope to act; its laws are not altered, but they operate under new conditions. Owing to the opposition to chemical affinity, the living tissues ever tend to decompose; as a weight that has been lifted tends to fall.

But the living structures are not the only instances, in nature, of bodies which tend to decompose. There are several in the inorganic world: such are the fulminating powders (iodide or chloride of nitrogen, for example), which explode upon a touch. There is a strong analogy between these and the living tissues. In each case, there is a tendency to undergo chemical decomposition; in each case, this decomposition produces an enormous amount of force. Explosive powders may be compared to steam that has been heated under pressure, and which expands with violence when the pressure is removed. The tendencies of these bodies have been coerced by some force, which is thus latent in them, and is restored to the active state in their decomposition. This is the point of view from which the living body, in respect to its power of producing force, should be regarded. The chemical tendencies have been resisted or coerced, and are, therefore, ready, on the slightest stimulus, to come into active operation. And the "functions" are effected by this operation of chemical force upon the various adapted structures of the body. The animal is a divinely made machine, constructed, indeed, with a marvellous delicacy, perfection, and complexity; and depending upon a power, the vital modification of force, which it is wholly beyond our skill to imitate, but still involving, in the laws of its activity, no other principles than those which we every day apply, and see to regulate the entire course of nature.

We speak of "stimuli" to the vital functions-of the things which stimulate muscular contraction, or stimulate the nerves. What is the part performed by these? They are what the spark is to the explosion of gunpowder; or what the opening of the valve that permits the steam to pass into the cylinder, is to the motions of the steam-engine. They do not cause the action, but permit it. The cause of the muscular motion is the decomposition in the muscle, as the cause of the motion of the piston is the expansion of the steam; it is the relaxing of the tension. In the muscle, the chemical affinity on the one hand, and a force which we will call, provisionally, the vital force on the other, exist in equilibrium ; the stimulus overthrows this equilibrium, and thus calls forth the inherent

tendency to change of state. Magnets lose for a time their magnetic property by being raised to a red heat; if, therefore, to a magnet holding a weight suspended heat enough were applied, it would permit the fall of the weight. It is thus the stimulus "permits" the function.

So one of the most perplexing circumstances connected with the phenomena of life becomes less difficult to understand; namely, that the most various and even opposite agencies produce, and may be used by us to produce, the same effects upon the body. The application of cold, or heat, or friction, alike will excite respiration. Any mechanical or chemical irritation determines muscular contraction, or will occasion in the nerves of special sense their own peculiar sensations. These various agencies operate, not by their own peculiar qualities, but by disturbing an equilibrium, so that the same effect is brought about in many ways. A sudden change is the essential requisite. As almost any force will cause a delicately balanced body to fall, so almost any change in the conditions of a living body, if it be not fatal to its life, will bring its functional activity into play. Anything that increases the power of the chemical tendencies, or diminishes the resistance to them, may have the same effect.

To recapitulate: Chemical affinity is opposed, and delicately balanced, by other force in the organic body (as we oppose forces in a machine; the elasticity of heated steam by the tenacity of iron, for example); and this affinity coming into play-spontaneously or through the effect of stimuli which disturb the equilibrium-is the secret of the animal functions. The body is not in this respect peculiar, but is conformable to all that we best know and most easily understand. The same principles are acted upon by every boy who makes a bird-trap with tiles and a few pieces of stick: here is the opposition to gravity, the equilibrium of force and resistance, and the unfortunate bird applies the stimulus.

But if the case be so simple, why has it not always been presented so? Why has it been conceived that the living body had an inherent activity peculiar to itself? And why especially has the decomposition of the body been represented as the result, and not as the cause, of its activity? Many circumstances have contributed to make this problem difficult of solution. In the first place, if the animal is like a machine in some respects, in others it is strikingly unlike one. All machines consist of two distinct parts: the mechanism, and the power. First, men construct the boiler, the cylinder, the levers, the wheels, all the parts and members of the steamengine, and then they add the water and the fire: first, they arrange the wheels, the balances, the adjustments of the watch, and then they bend the spring. In the body these two elements are united, and blended into The structure itself is the seat of the power. The very muscles, that contract, decompose; the brain and nerves themselves, in their decay, originate the nervous force. It is as if the wheels of the steam-engine were made of coal, and revolved by their own combustion;* or as if the watch

one.

The catharine-wheel is an instance of this very thing: structure and power united. But the firework is not renewed as it decomposes; the "nutrition" is wanting. VOL. II.-NO. 7.

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