Popular Science Monthly/Volume 15/October 1879/The Source of Muscular Power



THE question of the source of muscular power is essentially a question concerning transformation of energy. The most characteristic distinction between plants and animals is, that the former appropriate force from outside themselves, from sunlight, and store it up as potential energy in the various complex compounds which they form in; while animals draw their supplies of force entirely from those compounds in which it has been stored up by plants, and from which it is set free again when they are decomposed in the organism.

In a word, the plant converts the actual energy of the sunlight into the potential energy of organic compounds, the animal converts the potential energy of the organic compounds into actual energy, which manifests itself as heat, motion, electricity, etc.; in the plant the spring is coiled up, in the animal it uncoils, exerting an amount of energy equivalent to that which coiled it. One of the forms which this energy takes on is that of muscular motion, which we thus trace back to the potential energy of food, and through this to that great source of all energy to our earth, the sun.

We are not, however, satisfied with knowing in this general way that it is the food we eat which serves as a vehicle to convey to us our needful supply of sun-force. We were already acquainted with the necessity for food, but we wish to know which of the ingredients of our food performs this function, or, if all do it, which one performs it to the best advantage.

Until a comparatively recent date it was assumed unhesitatingly that the albuminoids—that is, bodies like albumen (white of egg), fibrine (muscular fiber), caseine (the basis of cheese), etc., which contain the element nitrogen as a characteristic ingredient, and which we shall designate collectively as proteine—were the proximate source of muscular power. It was taught that work was performed by means of an increased oxidation of the fibrine, of which the muscles are largely composed, and that the proteine of the food served to repair the wear thus caused. This view is still found in many especially of the smaller text-books of physiology, and seems to be the one generally current. Even so eminent a physiologist as Professor Austin Flint, Jr., has recently devoted a small book ("The Source of Muscular Power," D. Appleton & Co., 1878) to its defense; but nevertheless it was never founded upon experimental evidence, and has now been rendered untenable in its original form.

Karl Voit, of Munich, was the first to make exact experiments on this subject, and in 1860 he published the results of his researches, which showed conclusively that, contrary to the then generally accepted theory, muscular exertion did not increase in the least the amount of proteine decomposed in the body, although it was accompanied by a large increase in the amount of non-nitrogenous matters oxidized. This fact was immediately accepted by many physiologists as a proof that the commonly received view of the source of muscular power was incorrect, and that that power was in reality derived from the non-nitrogenous components of food—its fat, starch, sugar, etc. According to them, the muscles are, like a steam-engine, simply an apparatus for the transmission of energy furnished by some other substance, while the fat, etc., is the fuel of the living machine.

Voit and his followers, on the contrary, still hold that proteine is the proximate source of muscular power, though their views have naturally been materially modified by the experimental results just mentioned.

Voit compares the constant decomposition of proteine which goes on in the body to the constant flow of water in a stream. A mill situated by the stream may use the whole power of the water, a half, a quarter, or any desired fraction, without in the least altering the amount of water running past. So in the body the decomposition of proteine, which is the source of power to the muscles, goes on constantly, independently of whether the energy which is set free takes the form of motion or appears in some other shape.

These are, in outline, the views of the two schools into which physiologists are divided upon this point. Professor Flint, in his book already referred to, advocates, and seeks to sustain by experimental evidence, a theory which may fairly be said to have been abandoned by both sides; and a review of his book, which appeared in this journal in April, 1878, having given some prominence to the subject, a brief review of the present state of our knowledge upon it may not be uninteresting.

It will facilitate an intelligent comprehension of the matter to preface our study of the main question with some explanation of the means by which our knowledge of the amount of nitrogenous and nonnitrogenous matter decomposed in the body is gained, and by some considerations regarding the effect of the kind and quantity of food upon the nutrition of the muscular system and the excretion of nitrogen.

The animal body may for our present purpose be regarded as consisting, besides water, of proteine and non-nitrogenous matter, chiefly fat: the latter contains the elements carbon, hydrogen, and oxygen; the former, in addition to these three, nitrogen. Both classes of matter are gradually oxidized in the body, and are finally converted into carbonic acid, water, and urea, the former of which is excreted through the lungs and skin, the latter through the kidneys, and the water partly by all three channels.

In the urea (together with small amounts of uric acid and other products) is contained the nitrogen resulting from the oxidization of the proteine.

It has been established, by an overwhelming mass of evidence, that all the nitrogen which leaves the system does so in the urine, and that the amount of this element in the latter is an accurate measure of the amount of proteine destroyed in the body. A determination of ureal nitrogen thus informs us of the amount of albuminoids oxidized; while a determination of the amount of carbon excreted in carbonic acid and urea, taken together, enables us, by a little calculation, to find the amount of fat oxidized.

By means of experiments conducted on this basis a tolerably full knowledge has been obtained of the effect of food upon the formation of flesh (muscular substance) and fat, and facts have been discovered which have an important bearing, both on our views of the origin of muscular power and on the precautions necessary in experimenting upon this subject. The earliest workers in this field were Bidder and Schmidt, followed by Karl Voit, in conjunction, first with Bischoff and later with Pettenkofer.

One result of these researches has been to demonstrate that the consumption of proteine in the body is determined by the amount of it present in the food. If the food contains but little proteine, but little is oxidized in the body; if more be added, the consumption of it in the vital processes promptly increases, and within at the most three or four days comes into equilibrium with the supply, or very nearly so.[1]

Another important point is the distinction, first introduced by Voit, between what he calls circulatory and organized proteine. He has shown, by experiments which it would take too much space to describe, that the proteine of the body exists in two states: first, as organized proteine, which is comparatively stable; and, second, circulatory proteine, which exists in much smaller amount than the other, and undergoes a much more rapid decomposition in the body. The first effect of albuminoids in the food is to increase the amount of this circulatory proteine and the rapidity of its decomposition, and it is in this way that the consumption of proteine in the body is, as has just been stated, determined by the supply of albuminoids in the food. The production of organized proteine, which Voit supposes to constitute the muscular tissue, is, on the contrary, much less rapid, it being slowly formed from the circulatory proteine under proper conditions.

Some authorities dispute the correctness of the names circulatory and organized proteine, but there is no dispute as to the fact, shown by his experiments, that most of the proteine of the body exists in a comparatively stable form, while a small portion, dependent in amount upon the supply in the food, is being continually and rapidly oxidized and furnishes most of the nitrogen eliminated through the kidneys. We might compare the stock of circulatory albuminoids in the body to a mass of water contained in a vessel with a small aperture in the bottom. If there is no supply, it quickly runs out. If a small stream of water be let in at the top, a small supply of water may be maintained in the vessel. If a larger stream be admitted, the depth of water in the vessel will at once begin to increase, but at the same time the pressure on the bottom, and consequently the rapidity of the outward flow through the aperture, increases, and outflow and inflow soon come into equilibrium. If the supply be diminished, the level of the water sinks till the hydrostatic pressure causes the outflow to again equal the inflow.

Voit's results have been abundantly confirmed by other observers in experiments on various animals, including man, and must be regarded as fully established, whatever view we may take of the interpretation put upon them by their author. In Professor Flint's book, however, we fail to find any reference to these discoveries, though they have, as we shall see, a most important bearing on his own experiments. He does, indeed, mention the similar but less complete results of Lehmann (p. 35), and states (p. 41) that "the change of the normal diet to a regimen of non-nitrogenous matters alone of itself diminishes very largely the excretion of nitrogen"; but the more recent results of Voit and others are passed over in silence, though on pages 26–28 he mentions some facts and introduces some considerations which go to show the correctness of these results.

Coming now to the main question, we follow the general order of Professor Flint's book, and consider, first, the effect of muscular exertion upon the metamorphosis of matter in the body as shown by the excretions, and, second, the conclusions which can be drawn from these effects as to the proximate source of muscular power.

According to Professor Flint (p. 40), the experiments of Fick and Wislicenus in 1866 constituted "the starting-point of the new theory of the origin of muscular power." This, however, can hardly be said to be the case, at least as regards the experimental evidence on which that theory is based. We have already stated that Voit was the first to seek for evidence of the truth of the views held by his predecessors, and his experiments, as also those of E. Smith, antedate those of Fick and Wislicenus by some six years. Not only so, but the theory itself had been broached before Fick and Wislicenus made their experiments, as may be seen from their paper on the subject. At the same time their results gave it a powerful impulse and won for it more general attention.

Voit's conclusions have been fully corroborated by numerous and able investigators,[2] and are at present accepted by the great majority of physiologists; and we naturally expect some reference to them in a critical discussion of this question. We find, however, no mention of them; we are left to infer that the experiments of Fick and Wislicenus are the chief basis for the conclusion that work does not increase the elimination of nitrogen in the urine, and the author enters into a criticism of these experiments which is groundless, since it mistakes entirely their object.

The experiments of Fick and Wislicenus were not designed to show that work did not increase the destruction of proteine in the organism, but that the latent energy of the amount destroyed was insufficient to account for the work done. For this purpose they ascended an Alpine peak of known height, carefully determined the amount of nitrogen excreted during the ascent, and calculated from this the amount of proteine destroyed. Their diet before and during the performance of the work contained no proteine. On reckoning the amount of latent energy contained in the proteine destroyed, as shown by the quantity of heat which it would have yielded if burned, they found this energy to be insufficient to raise the weight of their bodies to the height of the mountain, and hence they conclude that the lacking energy at least must have been derived from non-nitrogenous materials.

Their experiments were well designed for the purpose intended, and the criticisms of Professor Flint (pp. 40–43) that the decrease in the excretion of nitrogen during and immediately after the work was due to the abstinence from albuminoid food, and that no comparison of rest with work was made, while doubtless well founded, do not touch the point at issue, viz., that a certain amount of work was performed and a certain amount of proteine destroyed, and that the latter was not, according to their calculations, sufficient to yield the amount of force actually exerted. The only grounds upon which the validity of their results can be successfully disputed are either that the principle of calculation employed by them or their data as to the heat of combustion of proteine were erroneous. We shall return to this point later. It may be added, in regard to the experiments of Voit and the other investigators in this field, that they are free from the failings which Professor Flint finds in those of Fick and Wislicenus, and also of Parkes.

The experiments of Dr. Parkes, which Professor Flint apparently regards as at least partially sustaining the view which he advocates, show in the great majority of cases either no increase or a slight decrease of the excretion of nitrogen as a consequence of work, and Dr. Parkes himself expressly says ("Journal of the Royal Society," vol. xix., p. 349): "The result of both series was, so far, to confirm the experiments, which show that the changes in the nitrogen of the urine. . . are small in extent, and afford no measure of the work."

Professor Flint's chief reliance, however, seems to be the experiments made in 1876 by Dr. Pavy, and published in a series of papers in "The Lancet," and his own experiments made in 1870 ("New York Medical Journal," June, 1871).

These two series of experiments differ decidedly, both in method and results, from those heretofore mentioned, both of them showing, according to their authors, an increased elimination of nitrogen through the kidneys as a result of muscular exertion. They were made upon two pedestrians, Perkins and Weston, during the performance of various feats of pedestrianism, and hence under conditions that excluded an exact measurement of the amount of work performed. Unfortunately, also, they could not, from the nature of the case, be made with that rigorous control of all the conditions of experiment which is essential in such researches; and they suffer under various sources of inaccuracy which materially lessen their value.

In the first place, no attempt was made to regulate the diet of the two men; they ate what and when they chose. In most experiments on this subject it has been considered necessary to employ a perfectly uniform diet as regards nitrogen, and an instructive example of the pains taken to insure it may be found in a paper by Voit and Pettenkofer in the "Zeitschrift für Biologie" (1866, p. 466). In the case before us, however, the investigators contented themselves with weighing the food eaten and estimating its contents of nitrogen. We have seen that Professor Flint elsewhere insists upon the importance of the food eaten in its effects on the excretion of nitrogen, but, both in his own experiments and Dr. Pavy's, there were, according to their own estimates, great variations in the amount of nitrogen ingested from day to day, as, for example, 65·68 grains and 161·72 grains, or, on another occasion, 522·42 grains and 871·92 grains on two successive days in Dr. Pavy's experiments, and 144·70 grains and 383·04 grains in Professor Flint's.

Such great and sudden variations as these could not but impair the accuracy of the experiments, and cause corresponding fluctuations in the amount of nitrogen excreted, as has been sufficiently shown by the investigations of Voit and others already alluded to, and the results bear testimony that such was the case.

Furthermore, not only did the quantity of nitrogen ingested from day to day vary, but even these varying amounts were not accurately determined by analysis, but simply, with a few unimportant exceptions, estimated from the average composition of similar articles as given by Payen. Neither Dr. Pavy nor Professor Flint appears to have even taken the trouble to estimate the water of the various articles of food, but to have simply weighed them in the fresh state—a fact which alone deprives the results of all claim to strict accuracy, since the water content of such articles as fresh meat or bread, for example, is quite variable, and the proportion of nitrogen in the fresh substance of course varies correspondingly. While such a method may give an approximation to the truth, it is impossible that, when applied to such a varied diet as that taken in these experiments, it should give results of scientific exactness.

The estimations of the ureal nitrogen appear to have been made after approved methods, and are to be assumed to be correct; but, even if we assume the accuracy of the estimates of nitrogen in the food as well, the results of Dr. Pavy do not show what he claims for them. They do, indeed, show that there was an increase in the average daily excretion of nitrogen during work over that during rest of 194·12 grains, and, at the same time, an increase of 201·63 grains in the average amount of nitrogen daily ingested. The only conclusion which can be drawn from these figures is, that during work more nitrogen was excreted because more was taken in the food. That muscular exertion caused any increase in the excretion of nitrogen we have no evidence.

With Professor Flint's experiments the case is somewhat different. There the amount of proteine taken in the food was considerably less during work than during rest, while the excretion of nitrogen remained about the same, so that the relative excretion was increased. This, Professor Flint claims, shows that the work performed was accomplished at the expense of muscular tissue, which was destroyed and caused the increase in the relative excretion.

Were the data as to food more exact, this might be the case; but, as it is, the result seems to need further confirmation before it can be accepted.

The only other similar result, so far as we know, is one recently obtained by E. von Wolff in experiments on the horse; but, having access only to a brief abstract, we are unable to judge of the accuracy of the work, though from the high reputation of this investigator it is to be assumed that it was executed with every precaution. It was found that an increase of the work performed was accompanied by an increased excretion of nitrogen in the urine; but the author reserves the details of his experiments till further observations shall have confirmed or disproved their results, and at present, until the subject has been more thoroughly investigated, we must follow the preponderance of evidence, which is most decidedly in favor of Voit's result, viz., that work does not increase the destruction of proteine in the body and the consequent excretion of nitrogen through the kidneys.

Thus far we have simply been considering experimental results, without regard to the conclusions to be drawn from them; we now come to their interpretation, and here it must be admitted, at the outset, that the data now at command are not sufficient to enable us to solve the problem of the source of muscular power. But, though we do not know precisely what the proximate source of muscular power is, we are able to indicate with tolerable certainty the direction in which an answer to this question is to be sought, and to say that certain conclusions have a high degree of probability.

It would seem at first thought that if, during work, the oxidation of non-nitrogenous matters in the body increases, while no more proteine is destroyed than during rest, the non-nitrogenous matters must be the source of the power exerted. This appears to be Professor Flint's view, as indicated by several passages in his book; but, though it may be a probable conclusion, it is by no means a necessary one. We have already mentioned the fact that Voit and his followers still consider the constant decomposition of circulatory proteine which goes on in the body to be the source of muscular power, comparing it to a constantly flowing stream, the energy of which may be converted at will into motion, or be allowed to take the form of heat; and there is nothing in the experimental results above adduced to forbid this interpretation.

It has been shown by Voit and Pettenkofer to be at least very probable that proteine in its decomposition in the body takes up the elements of water and splits up into urea and fat; and it is easy to show by calculation that 100 parts of proteine could produce in this way 51·4 parts of fat. This process, now, takes place during rest, and it is quite conceivable that during work the proteine is decomposed completely into carbonic acid, water, and urea, and that thus the latent energy which would otherwise be stored up in the fat is applied to the production of motion. If this were shown to be the case (and it seems not improbable that something similar to it actually takes place), it would become largely a question of nomenclature whether we should regard the proteine or the fat which is formed from it as the source of muscular power. For ourselves, we believe that the truth will eventually be found to lie between the two extreme views now advocated, and that muscular force will prove to have some such origin as that above indicated.

At the same time there are certain facts immediately to be considered which show that the process is by no means so simple as that just sketched.

If we turn from the study of the effects of muscular exertion to that of its conditions, we shall get much new light, and be helped to a more rational judgment of the theories as to its source. Presupposing the existence of a healthy and well-developed organism, we may specify four conditions as, from our point of view, the most important:

1. The facts of common experience appear to show unmistakably that a liberal supply of proteine in the food is one of the conditions of any sustained muscular exertion. This, however, does not necessitate the conclusion that the proteine is the source of the power exerted: its decomposition, as we have seen, goes on independently of muscular exertion, and may be regarded as simply one of the conditions of the healthy activity of the muscles.

2. The largely increased excretion of carbonic acid and water during work indicates a necessity for a liberal supply also of the nonnitrogenous constituents of food. At need, however, this demand may be supplied by the albuminoids of the latter, or perhaps by fat already formed in the body.

3. An essential condition of continued activity of the muscles is the constant removal from them by the circulation of the chemical products of their action. Certain of these products, notably lactic acid and acid potassium phosphate, if allowed to accumulate in the muscle, produce the sensation of weariness, and shortly incapacitate it for further action. If they be removed, either by the blood or by injection of a weak salt solution, the muscle is again capable of work; while, if they be injected into a fresh muscle, they produce the same effect as if naturally formed there. The same or similar processes go on in the muscle after death, and the rigor mortis is caused by the solidification of the jelly-like myosin, which is also one of the products of the action.

4. A most important condition of muscular activity is found in the capacity which the body has to store up oxygen in itself during sleep, to be used later in the waking hours. This capacity was discovered by Voit and Pettenkofer in experiments on men, and has been confirmed by Henneberg's experiments on oxen. More carbonic acid is excreted by day than by night, since more work is then done. But at the same time less oxygen is taken into the body in the daytime than during the night. For example, in one of Voit and Pettenkofer's experiments, for every 100 parts of oxygen which entered the system in the daytime 175 parts were contained in the carbonic acid excreted, while in the night the same relation was expressed by the number 58. When work was performed the difference was still greater. This and similar experiments show plainly that a large part of the carbonic acid excreted is formed at the cost of oxygen previously laid up in reserve, and that the increased rapidity of respiration during work is not for the purpose of supplying more oxygen, but of removing the carbonic acid.

It has been also shown that the amount of oxygen that can thus be stored up in health is proportioned to the amount of albuminoids in the food, and this is another indication of the importance of these bodies in the production of muscular power.

The necessity of this storing up of oxygen is strikingly shown by experiments on two diseases in which the patient is almost incapable of muscular exertion, viz., diabetes mellitus and leukæmia lienalis. In these diseases the total excretion and the total amount of food are not much different from those in health; but there is no such storing up of oxygen as in the healthy organism, and there is also an almost entire lack of strength.

This fourth condition is, for our present purpose, the most interesting and important of all. It shows that work is not produced by the direct oxidation of food materials by the oxygen of the blood, but that the muscles themselves contain a store of latent energy which the will can set free at pleasure, independently of oxygen, while the blood serves to wash out the waste products and gradually to renew the supply of force during those periods of rest of which this fact explains the necessity.

That the seat of this latent energy is in the muscles is shown by the fact that they are capable of contraction for a time after their blood-supply has been cut off, or even after their removal from the body. A frog's heart, when removed from the body and freed from all blood by injection of a weak solution of salt, will continue to beat for hours, and the whole animal under the same circumstances moves, leaps, and behaves in short like a living animal. Agassiz relates that on one occasion he captured a shark which fought as long and fiercely as is usual with these animals, but which, when finally secured, was found to have its gills eaten through by parasites, and almost all its blood replaced by sea-water. (Liebig.)

Like a bent spring the muscle contains a certain amount of potential energy, which the will can use at pleasure; but when the supply is once exhausted, when the spring has lost its tension, a further supply of force from without is necessary before more work can be performed.

These facts furnish an important clew to the source of muscular power. The experiments of Voit and Pettenkofer show that while storing up oxygen during rest the organism is laying up a store of force to be used later; while those of Henneberg connect this storing up of oxygen with the supply of albuminoids in the food, and render it highly probable that it is accomplished by their means.

Two hypotheses as to the function of the albuminoids as agents in the production of muscular power at once suggest themselves. The first is, that they simply serve as reservoirs of oxygen, which latter is used at will to burn the non-nitrogenous parts of the food, the result being work, heat, and an increased excretion of carbonic acid and water. This would be the view of those who consider the carbhydrates and fats as the source of muscular power, and its simplicity renders it attractive. It must be noted, however, that it requires us to look upon the non-nitrogenous materials oxidized as part of the muscle, since the latter can perform work independently of the circulation of blood through it.

A second hypothesis, however, less simple and easily grasped than the first, is considered by many high authorities to accord more closely with the facts of the case and with our general conceptions of vital activity.

This hypothesis supposes that during rest some of the substances of the muscle-cells decompose into simpler compounds, and in so doing set free their latent energy, which energy, instead of appearing as heat, etc., is used to build up out of other constituents of the cell a still more complex compound containing more potential energy than its components, just as one portion of society may acquire wealth at the expense of another portion, with no increase of the total wealth of the community.

The substances which are thus "synthesized" are proteine, an unknown non-nitrogenous matter from the blood and oxygen; the hypothetical compound thus formed accumulates to a certain extent in the muscle, and, when the latter is called on to perform work, splits up, yielding carbonic acid, water, and other non-nitrogenous matters, and proteine or some similar compound, and giving forth the amount of force which was required to form it. The non-nitrogenous substances which are formed are supposed to be rapidly excreted; while the nitrogenous product, instead of undergoing further decomposition, is used over again to re-form the hypothetical substance.

This view has much in its favor. Various syntheses like that above outlined are known to take place in the body; and, moreover, all the facts seem to indicate that muscular force originates in a splitting up of some substance in the muscle, accompanied by the liberation of force, rather than by any process of oxidation in the ordinary sense of the word.

The hypothesis explains the object of the storing up of oxygen in the body during rest, and its connection with the laying up of a reserve of force: the oxygen enters into the supposed complex compound much as the nitric-acid radicle enters into nitro-glycerine or gun-cotton—it is held in a state of unstable equilibrium, ready to enter into new and simpler relations with its neighboring atoms, and to set free the force by which it was placed in its unstable position. The hypothesis explains also that necessity for albuminoids in the food of the laboring animal which practical experience has shown to exist, as well as the fact that there is no greater excretion of nitrogen during work than during rest; the proteine serves as the basis for the alternate synthesis and analysis which constitute what might be called the atomic mechanism of muscular activity without itself being destroyed. Furthermore, it shows why we need rest after work; in the first place, the circulation must have an opportunity to remove those waste products which accumulate in the working muscle faster than they can be carried off, and in the second place a fresh supply of force must be stored up in the way described before it is ready to be used at the command of the will.

Thus this theory explains all the facts now known, and, while it is but an hypothesis, it is still based on the "scientific use of the imagination," and indicates the direction in which we may confidently look for an advance of positive knowledge.

If it be true, much of the current discussion upon the source of muscular power is but a "strife of tongues"; both proteine and nonnitrogenous substance are necessary, and an inquiry as to which is the source of the power would resemble an inquiry as to whether the explosive force of nitro-glycerine was derived from the glycerine or the nitric acid used in its manufacture, and would be a question of metaphysics rather than of natural science.

It might be asked, since this is a question of transformation of energy, why we do not apply the law of the conservation of energy, and from the heat of combustion of the various elements of food calculate their value as reservoirs of force by Joule's formula. This has been frequently attempted, notably by Fick and Wislicenus in their experiments already alluded to. These investigators showed that the amount of force contained in the proteine which was destroyed in their bodies during the ascent of the Faulhorn was insufficient, if wholly converted into motion, to raise their bodies to the height of the mountain.

Various other attempts at the same sort of calculation have been made, with more or less of care and insight: we may mention here those of Dr. Pavy, which rest on several assumptions of questionable accuracy; and those of Professor Flint, made upon the same plan, with the object of showing the worthlessness of Dr. Pavy's—an object which he has doubtless attained.

It would carry us too far to discuss here the value of these results, and we must content ourselves with two general statements: 1. The heats of combustion of the various food-substances, which serve as the foundation of all such calculations, have not yet been determined with sufficient accuracy to render those calculations demonstrative. 2. Even if it were shown that the results of Fick and Wislicenus are correct, and that the albuminoids destroyed during work are not sufficient to supply all the force exerted, this in no way invalidates our hypothesis, since the latter does not place the source of muscular power in the albuminoids alone, but in the joint action of these and of non-nitrogenous matters.

It will be seen that the foregoing views as to the origin of muscular power are in some respects in substantial accordance with those of Professor Flint. Like him we hold that the source of muscular power is to be sought in the muscles themselves, and not in any burning of the constituents of the food in the blood or the juices of the body. Muscular power, we believe, does not have its immediate origin in oxidation but in the splitting up of an unstable compound into simpler ones.

We differ from him, however, both in regard to the effect of muscular activity upon the destruction of proteine in the body and in regard to the conclusions to be drawn from these effects. Professor Flint claims that work increases the amount of proteine destroyed; we believe we have shown that neither his own experiments nor those of Dr. Pavy are sufficient to prove this, and that the preponderance of evidence is altogether in the other direction.

He says further (p. 31): "In other words, is the muscular substance an apparatus for transforming the force locked up in food into power, or are the muscles themselves consumed, the elements of food being used for their repair? These questions may be resolved by little more than a single experimental line of inquiry: Does physiological exercise of the muscular system increase the elimination of nitrogenized excrementitious principles?"

Were these questions capable of being resolved in this simple manner, their answer would be just the reverse of that which Professor Flint gives to them; but we have already seen that such researches are entirely inadequate, of themselves, to settle the matter, and that very different considerations must be attended to in order to attain that end.

Some of these considerations we have endeavored to present, as clearly as might be, in the foregoing pages, while pointing out what seems to us the false method by which Professor Flint, in his very interesting book, has sought to maintain a conclusion which itself is doubtless correct, viz., that muscular power originates in vital actions taking place in the cells of the muscles themselves and not in a simple oxidation of food-constituents. We can not but regret that this fact, which he so clearly appreciates and states, should be supported by invalid reasoning, and that the sanction of a name so eminent among American physiologists should be given to views which do not accord with the results of the best and most recent investigations on this subject.

  1. Voit's experiments were made on dogs. With herbivorous animals the same law holds, but the change is not so rapid.
  2. Among these confirmatory results may be mentioned, as having historical as well as scientific interest, those obtained in 1867 by T. R. Noyes, M. D. ("American Journal of Medical Science," October, 1867), then a student in the Yale Medical School, which in the main agreed with those of Voit.