LECTURE V
Source of muscular energy—
Before beginning this lecture, I will show you an experiment which Professor du Bois Reymond calls the muscle dance. You see the muscle connected with the interrupter (see Fig. 11, p. 30), so that when it contracts it breaks the primary circuit of the induction coil and also rings the electric bell. On the scale-pan below the interrupter, I have placed a heavy weight, which at once stretches the muscle when it begins to relax. The relaxation of the muscle, however, again closes the primary circuit, and the muscle receives a shock from the secondary coil. This again causes it to contract and to ring the bell. Again it relaxes and gets another shock. Thus the muscle breaks the circuit by its contraction, and forms it by its relaxation, and each time it does so it gets a shock. It thus dances to its own music, as you now see and hear.
We have hitherto studied muscular movements caused either by a direct electrical stimulus or by the action of a nerve. There are, however, rhythmic movements of muscular substance. You see here a frog's heart attached to an apparatus by which it can be fed with blood, and you see it beating with great regularity. The heart is caused to work a little manometer, and on the mercury in one limb of the manometer I have placed a little glass rod bearing a flag, so that you may see the flag moving up and down with each beat of the heart. In this way a heart can be kept alive for hours, and we can estimate the amount of work it is able to perform. This also illustrates a method by which physiologists can examine the influence of substances on the heart. Thus we might feed the heart with blood free from any poison and note how it worked. Then we might feed it from this other tube with blood containing a small percentage of the substance to be examined, and again note the effect. By comparative 
Fig. 62.—The frog-heart apparatus, as devised by Professor Kronecker of Berne. g, the heart fixed on the end of a tube which has two branches. One branch, to the left, 'd, communicates with a stop-cock, a, by which the heart can be fed with blood either from the tube c or b. The other limb of the tube, to the right, passes to a small manometer containing mercury. On the longer limb of the manometer is a little glass rod, e f. When the heart beats, it cannot force the blood back to the tube b by d, because the stop-cock a is shut. It presses on the mercury in the manometer and raises e f. experiments of this kind, we can get valuable information of great use to the physician, and it is satisfactory to know that information thus obtained from experiments on the frog's heart has been found to agree with that got by experimenting on the hearts of warm-blooded animals. This is another example of how valuable to humanity the frog has been in the way of giving scientific information. It is not too much to say that each time your physician sees you when you are ill he brings to the study of your case knowledge that has been gathered for him by the physiologist from the frog. As we are in the habit of commemorating by monuments the services of those who have been benefactors to humanity, I know no animal,—no tiger, lion, or panther,—that better deserves a bronze statue than the humble frog. Such a statue in Trafalgar Square or on the Thames Embankment would not inappropriately mark our appreciation of the services he has involuntarily rendered to humanity.
Referring to rhythm, look for a moment at this experiment in which you see a muscle—the sartorius—beating rhythmically like a heart, because it is immersed in a fluid, called Biedermann's fluid, composed of common salt, alkaline phosphate of soda, and carbonate of soda and water. This striking experiment, which always is of great interest to me, shows how rhythmic movement may to some degree depend on nutritional changes going on in the muscle.[1]
We saw in last lecture that a muscle is a little laboratory in which chemical processes go on, and that the energy manifested by the muscle depends upon the activity of these operations. If a muscle is constantly throwing off effete matters arising from the wear and tear of its substance, and if it is always expending energy, fresh matter and fresh energy must be supplied to it. What is the source of supply? You naturally answer, the blood, and this answer is right. It is this fluid that brings to the, muscle the matters that it uses to build up its substance or the matters that it acts upon, as we may suppose a machine acting on something supplied to it. But a further question arises. Whence does the blood receive these new materials? Evidently from the food and from the oxygen we take into the blood by respiration. Food stuffs are then the source of the energy set free by the muscle.
A very little consideration shows us that animals live on food stuffs that are apparently very different from each other. An ox eats grass; a lion lives upon flesh; a man prefers a mixed diet, such as meat and potatoes. The diets of even the various races of mankind present remarkable differences. The native of Bengal lives largely on rice with a little fat; some Europeans, like the French and Italian peasantry, partake of a diet consisting almost wholly of vegetable substances; the Turkoman, in the steppes of Central Asia, consumes vast quantities of flesh; and the Esquimaux finds that a diet rich in fat best enables him to withstand the rigours of his severe and inhospitable clime. Men have found out by experience what suits them, and no doubt custom or habit has a great deal to do with the selection they make. Can we then compare their dietaries with the view of solving the problem how energy is to be obtained from substances so unlike as those I have mentioned? We get light on this question by examining a natural food, one upon which almost all men live in the earlier part of their existence, the food of childhood, milk.
I pour a glass of milk into a basin and warm it, adding a few drops of acetic acid. You see it quickly undergoes a change. Masses of curd make their appearance and float in a fluid of a yellowish colour, familiarly known as the whey. We filter it. The curd you see is a soft friable matter. It consists of a body called casein, of very complex chemical composition. If we gave it to a chemist to analyse, he would tell us that it contained carbon, hydrogen, oxygen, and nitrogen. Note particularly that it contains nitrogen; it is, as we say, a nitrogenous substance, and it represents the first constituent of every diet, which must contain a nitrogenous or, as it is termed, a proteid substance. Proteid bodies include such substances as we find, along with other matters, in white of egg, in meat, in wheat, in oats, in beans, in grass, and in many other well-known articles of diet. The casein. as I have said, represents the first essential constituent of a diet, proteid or nitrogenous matter.
Now let us examine the whey. It is sweet from containing a kind of sugar called milk-sugar. These sugar substances, when heated with an alkaline solution of a salt of copper, have the property of taking oxygen from the copper compound, changing it to one that is insoluble; and in effecting this change they also cause a change of colour. Thus you see, when I add to this solution of grape-sugar a blue solution of a copper salt (called Fehling's solution, an alkaline tartrate of copper) and heat it, the blue colour gradually disappears and a reddish substance, an oxide of copper (cuprous oxide) falls to the bottom of the glass. Applying the same test to the whey, we find proof of the existence of sugar in it. Now sugar is a compound of carbon, hydrogen, and oxygen. It contains no nitrogen, and hence it is called non-nitrogenous, to distinguish it from the nitrogenous or proteid group of bodies already referred to. As the oxygen and hydrogen in sugar are in the proportion by volume of one of oxygen to two of hydrogen, as in water, sugar is said to be a carbo-hydrate; or, as one might phrase it, it is hydrated carbon. Suppose, as 1 now do, we pour some strong sulphuric acid, which has a great affinity for water, upon a bit of lump sugar, you see the lump soon becomes a black mass of carbon or charcoal. This group of carbo-hydrates includes the various kinds of sugars, starches, and gums. Carbo-hydrates are always present in a diet. They abound in rice, sago, potato, and bread, and in vegetable matters used as foods.
I need hardly demonstrate to you that milk contains butter. Butter is a mixture of various fats, and as fats are soluble in ether, it is easy to make an ethereal solution of the fat of milk by shaking up milk with ether, after adding a little caustic potash to it and keeping it moderately warm. In this long tube you see a layer of ether holding fat in solution, floating on the top of the fluid. Fat also consists of carbon, hydrogen, and oxygen, but in proportions different from those in which these elements exist in the carbo-hydrates. It contains no nitrogen. Fatty matter must always exist in a diet suitable for sustaining life. Then we have in milk various mineral or saline substances, such as chloride of sodium or common salt, chloride of potassium (possibly), phosphates of soda and potash, phosphates of lime and magnesia, and the ash always shows traces of iron, although we are not acquainted with the exact condition in which iron exists in milk. Here is some ash of milk prepared for us by the chemists, and it would not be difficult to show you that these salts exist in it. Lastly, milk always contains water as the solvent for all the substances I have mentioned. Examine for a moment this table.
| Cow. | Goat. | Mare. | Dog. | Human. | Condensed Swiss. |
Condensed English. | |
| Proteid matter chiefly casein |
3.34 | 4.20 | 2.70 | 11.70 | 2.45 | 10.20 | 11.84 |
| Fat (butter) | 3.53 | 5.80 | 2.50 | 9.72 | 3.10 | 9.76 | 8.30 |
| Carbo-hydrate (sugar) | 4.75 | 4.94 | 5.50 | 3.00 | 6.70 | 51.02 | 50.79 |
| Salts | .75 | 1.00 | .50 | 1.35 | .30 | 2.32 | 2.00 |
| Water | 87.63 | 84.06 | 88.80 | 74.23 | 87.45 | 26.70 | 27.07 |
Milk, then, a typical food, contains proteid or nitrogenous matter in the form of caseinogen,[2] carbo-hydrate in the form of milk-sugar, fat as butter, saline matters, and water. All these so-called proximate principles must exist in any dietary that can keep an animal healthy and strong. The reason of this is, that if you examine chemically almost any of the tissues of the animal, you find that they are built up of the same kind of proximate constituents. Suppose a chemist analysed muscle, he would find in it proteid matters in the form of a substance called myosin, along with other albuminous bodies; carbo-hydrates would be represented by glycogen, a kind of animal starch, and by sugar; fats are there also; and if he burnt the muscle, an ash would be left containing the same salts as we found in milk. But the constituents in the food stuffs are not quite the same as those in muscle, and they are therefore subjected to many chemical processes in digestion by which they are first converted into stuffs that exist in the blood. From these stuffs in the blood the muscle builds up its- substance. Now all of these stuffs, from the scientific point of view, contain energy in what is spoken of as a potential state, that is to say, it is resting, ready to be set free, ready to do work, and when it is set free it may become, as it does become in the case of the muscle, mechanical energy and heat. This locked-up energy is liberated by one familiar process, that of burning. But burning is oxidation. The elements of the substance to be burnt are torn from each other, and they unite with the oxygen of the air to form simpler bodies. When this occurs, energy appears as heat. Similar phenomena occur in a muscle. The muscle needs oxygen and it needs food stuffs. It builds some of these food stuffs into its own substance, thus always making up for the "tear and wear" that goes on when it works, and it uses some of the others, effecting in some mysterious way chemical changes in them, always splitting them up into simpler bodies.
Let us think of the muscle as a little machine or engine. When it works, it is subjected to tear and wear, like any other machine, and by and by it would become unfit for work. This happens, as we all know, with any machine. The boiler plates of an engine get thinned, the pinions become slack, the wheels do not work so smoothly as at first, and a time comes when the engine becomes only old iron and is unfit for use. But while the same process of tear and wear goes on in a muscle, the muscle, being a living thing, has the power of self- repair. It is always engaged in mending itself, building up so as to make good the waste, and in this way for a long time it is able to work efficiently. The substances needed for building it up are brought to its own door by the blood, many of them ready made, and it takes them into itself and repairs the machinery. These substances for repair are no doubt the proteids, the carbo-hydrates, the fats, saline matters, and water. They all seem to be necessary for the upkeep of the muscle-substance.
But our little machine not only keeps itself in repair, but it can excite chemical changes in certain matters brought to it, and by these changes energy is liberated. There are strong grounds for holding that the carbo-hydrate matters are changed or altered in this way by the action on them of the living muscle-substance. The history of these carbo-hydrates is very wonderful. Entering the body mainly as starch, they are changed into sugar; then they pass into the blood and are carried to the liver; then they are reconverted into the animal starch, glycogen, and stored up in the liver for further use; lastly, the glycogen is again changed into sugar, either in the liver or in the muscles; and in the muscles the sugar is used up, being ultimately decomposed into carbonic acid and water. The splitting up of this sugar or carbo-hydrate in the muscle is, there is every reason to think, the main source of the liberation of the energy in the muscle. This view explains in particular the greatly increased consumption of oxygen and the greatly increased production of carbonic acid following active muscular exertion. At least a part of the carbonic acid is the waste product arising from the decomposition of the carbo-hydrate.
Now if the muscle receives no carbo-hydrate, or an inadequate supply of it, it does not follow that it will stop working. Experiment has shown the contrary. It will still work, using up the fat in the first instance; and if there is an inadequate supply of fat, it actually uses up proteid matter, or, in other words, it uses its own substance. If an animal is caused to work very hard, we do not find an increase in the excretion of the nitrogenous waste matters, as Liebig supposed; but, as I have said, an increase in the non-nitrogenous waste matter, carbonic acid. So long as carbo-hydrates or fats are freely supplied, no increase in nitrogenous waste matters follows prolonged muscular exertion; but if they are withheld, or if the muscular exertion is excessive, then there is an increase in the waste nitrogenous substances. This shows that when the little machine cannot give its output of energy at the expense of carbo-hydrate or of fat, it sacrifices a part of its own framework. The muscle engineer, when he finds himself short of the ordinary fuel, seizes hold of combustible portions of his own engine, as if he were determined at all costs to do the work required of him. This is only a somewhat fanciful analogy, but it gives us an insight into what probably occurs in a muscle.
An engineer is desirous, chiefly for the sake of economy, to get as much effective work as possible out of the engine he constructs. The engine is intended to do work by liberating mechanical energy; but part of the energy appears as heat, and the heat is of no use to the engineer. The engineer knows the amount of energy represented by the fuel he uses, and he can estimate the amount of effective mechanical energy he can get out of it by the best arrangements yet devised. We are told that the best triple expansion steam-engine, with the best arrangement of furnaces and boilers, gives back of effective mechanical energy only about twelve and a half per cent of the total energy in the fuel. This means that of every one hundred parts of energy supplied to the engine only twelve and a half parts are of any use, the remaining eighty-seven and a half parts being lost as heat. The case is worse when we consider an ordinary locomotive, by which only about four per cent of the total energy becomes effective. It is interesting to inquire how the muscle, considered as a little engine, compares with the best steam-engine.
So long ago as 1869, Professor Fick of Wtirtzburg stated that the amount of energy transformed into mechanical energy by a muscle was about thirty-three per cent of the energy in the food stuffs. In 1878, he announced that more accurate experiments had obliged him to reduce the estimate to twenty-five per cent. Fick's experiments were made on the isolated muscles of frogs, and to some extent were vitiated by the conditions in which the muscles were examined.
To illustrate what is meant by the work of a muscle, I show you here an interesting experi-
Fig. 63.—The work-gatherer of Fick. It may be called a muscle-winch or muscle-capstan.
ment carried out by an ingenious instrument devised by Professor Fick, and which he calls the Arbeitssammler, the work-gatherer. It is a little windlass or capstan, which you see is turned by a frog's muscle placed above it, and 
Fig. 64.—Arrangement of the muscle-capstan. The shadow was projected on the screen. The muscle was placed above the capstan and the nerve was irritated at regular intervals, say of one minute, by tetanising shocks lasting only for a short time, say three seconds. By means of the pulley placed below the stand bearing the capstan, the lifting of the weight (made of cork) with each contraction of the muscle was so large as to be readily seen. which I stimulate at regular intervals of time. You observe a little catch on the edge of the wheel, which keeps the wheel from going in the opposite way during the relaxation of the muscle. Consequently the muscle, as you see, winds up the weight. Now if we multiply the weight by the height through which the muscle has lifted it we get a measure of the work done. We speak of a foot pound, that is one pound weight lifted one foot in height, or we speak of a kilogrammetre, that is one kilogram lifted one metre in height. In like manner, in estimating the work of a muscle, we use the phrase gramme-millimetre, that is one gramme lifted one millimetre in height, or about fifteen grains lifted the one-twenty-fifth of an inch. You observe how easy it is to get a notion of what musclework means by the use of this beautiful instrument.
It has been found that the work actually obtained from a frog's muscle may be stated as follows: one gramme of muscle (that is about fifteen grains) will yield four gramme-metres of work. A gramme-metre is one gramme lifted one metre (a little over three feet). Four gramme-metres represent, then, fifteen grains lifted a height of twelve feet. This may seem a small amount of work, and it would be so if, in doing it, the gramme of muscle disappeared; but only an infinitesimal part of the muscle-substance is used up in the experiment, and the most careful weighing would probably fail in detecting the loss. Perhaps not more than the one-thousandth part of the fifteen grains of muscle has been used to lift that weight twelve feet high.
Chauveau, one of the greatest of living French physiologists, has recently reinvestigated the question by ingenious experiments on the muscles of living men in normal conditions. These experiments oblige Chauveau to reduce Fick's estimate and to give the total effective energy as only from twelve to fifteen per cent. Taking the total mechanical energy of a man instead of a muscle, some recent calculations of my own show an output of mechanical energy as a little over seventeen per cent.
It is evident, therefore, that, considered as an engine, a muscle is not much better as a transformer of energy than the best steam-engine now constructed, while it is inferior to certain gas-engines which are said to return as much as twenty per cent in the form of effective mechanical energy. The muscle, however, has this great advantage over any engine, that the heat it produces supplies one of the conditions of its very existence, the maintenance of a certain uniform temperature. Muscle-substance will only work within a limited range of temperature, and as the body is constantly losing heat by radiation, conduction, evaporation of sweat, and by other means, heat must be supplied. This comes mainly from the muscles. It would not be correct to say, however, that one of the final purposes of a muscle is to produce heat. It is not a heat-producing machine. The primary function of a muscle is to contract, and thus to do effective work; but by producing heat at the same time it becomes possible for the muscle -substance to do its work in the best possible conditions. This is only another illustration of that wise economy that we see in most, if not in all, of the arrangements of nature.
After considering the points discussed in this lecture, you will readily understand how it is that a muscle becomes tired. It becomes fatigued after continuous work. Fatigue means a diminished power of work. Up to a certain point, the substances produced in a working muscle are got rid of as quickly as they are formed and new materials are supplied for the repair of the muscle. There is thus a balance between the two processes. But if a muscle is made to work for a long period, or if it is excited to very frequent contractions, the waste products gather or accumulate in the muscle, and sufficient time is not allowed for the supply of reparative material. Muscle, like most other tissues, is richly supplied by a special set of vessels, of very minute size, called the lymphatics, which are for the purpose of draining away the excess of nutrient matter that has oozed out of the vessels, along with the waste matters that have been formed. If the waste stuffs are produced too quickly—such stuffs as carbonic acid, acid phosphate of potash, and the stuffs of a nitrogenous nature, such as kreatin and other bodies found in extracts of meat—the muscle becomes fatigued; it consumes less oxygen and produces a smaller amount of waste products.
It is very instructive to watch how a muscle behaves as it becomes tired. I have fitted up this beautiful experiment, as devised by Marey, to show you this. The pithed frog (entirely devoid of sensation) lies on this cork plate, and a thread from the tendon of
Fig. 65. Arrangement of apparatus for the demonstration of fatigue. a, recording cylinder; b, railroad carrying the myograph, c; d galvanic element; e, induction coil; f, key.
the gastrocnemius passes to a light lever writing on the surface of this drum. The cork plate, bearing the frog, is on a stand that moves by clockwork from left to right, so that the plate moves a little to the right during one revolution of the drum. Thus the tracings are kept from blurring, each successive curve being a little to the right, or, to put it in another way, a close threaded spiral is described round the drum. Now we shall irritate the nerve by an opening and a closing shock from the secondary coil of our induction machine, each shock coming always at the end of an equal interval of time. This we can arrange by attaching a wire to the axle of the cylinder, so that it stands out at right angles to the axle, and as the axle revolves, the wire dips into this trough containing mercury, thus closing the current of the primary coil of the induction machine, and the next instant the wire comes out of the mercury, thus opening the current of the primary coil. As you know, this will secure a shock from the secondary coil each time the wire dips into and comes out of the mercury. The current is led from the battery, through the primary coil, thence to the cylinder, thence through the wire into the mercury, thence back to the battery. Now notice the beautiful curve. It shows, first, the gradual lengthening of the period of latent stimulation, as indicated by each contraction beginning a little later than the one before it. You observe the gradual slope of the line joining the nings of the successive curves. Second, we find that at the beginning of the experiment the amount or amplitude of each contraction
Fig. 66.—Consecutive tracings of the contractions of the gastrocnemius muscle of a frog, showing the effects of fatigue. Chronograph, 100 vibrations per second. Marey.
slightly increases, or in other words, by successive stimulations, the muscle gets into good working trim. In the curve in the diaoram this increase is seen, as the curves at the top of the diagram are higher than those at the foot. Fatigue, so far as amplitude of contraction is concerned, has scarcely begun. By and by, however, as the muscle becomes fatigued, the amount of contraction diminishes, until the muscle does not contract at all, but the duration of the contraction increases throughout the whole contraction. The muscle is gradually losing time in doing its work. When does it lose time? In contracting or in relaxing? You observe the slope of the successive curves; the way in which they open out, as seen if you study each from the bottom to the top of the diagram, shows that it loses time during relaxation. During fatigue a muscle after contracting returns more slowly to its original length.[3]
These results are consistent with our experience. After a thirty mile walk, we feel unwilling to take each step; it is only by a strong effort of the will that we force the muscles to contract. Like jaded horses, they require the whip and spur. The muscular contractions required for each step, however, are not shorter in duration. When the muscles do respond they contract as usual, perhaps not to so great an extent, but then they relax slowly, and we wearily drag our limbs onwards. I do not say that fatigue is entirely in the muscles. They communicate with headquarters and they telegraph their wearied condition to the executive, and the executive also becomes tired, partly by receiving these messages from the muscles, and partly by having to stimulate the flagging muscles to a much greater extent than when they are fresh and active. Using the whip and spur may weary the rider while they stimulate the steed.
There is one other peculiarity of the muscle, considered as a machine, that distinguishes it, to some extent at least, from all other machines. You are all aware that in a great factory containing complicated machinery it does not pay to allow the machinery to stand still, even suppose it may be working at a daily loss to the manufacturer. It may so happen, and unfortunately it not unfrequently happens, that bad trade compels a manufacturer to produce his articles at a loss, and you might think it would be prudent on his part to stop his machinery. But this would probably be an unwise step, both because he might lose the market, and still more because his machinery would deteriorate by standing idle. The manufacturer therefore prefers to hear the whirr and roar of his machines, although he may feel that they are working at a dead loss to him. He does not expect that the work will improve his machines, but he is sure that it will keep them in good condition.
The muscle-engine, in like manner, deteriorates if it is allowed to stand idle. Sometimes this occurs in disease. In paralysis the limb cannot move, and the inactive muscles waste, become thin and flaccid, and undergo curious molecular changes, converting the muscular matter into fatty-like particles. The wise physician, however, knows this, and, like the manufacturer, he keeps the machinery running. He does this by stimulating the muscles by electricity and causing them to contract. The electricity supplies the stimulus that the nerves cannot give, and the physician keeps up the strength of the muscles and hopes for better times. We often, however, fall into the bad habit of allowing our muscles to be inactive, and the result is they become weak and attenuated. Exercise is needed. Run the machinery and you obtain the wonderful result that the living machine improves in strength and size. The mere act of making the muscle work develops its powers. It grows stronger and thicker, and it works with greater precision and effectiveness. Hence the value of athletic exercises, if carefully carried out. They should not, however, fall on one muscle or one set of muscles exclusively. If they do, these muscles, instead of being benefited, suffer from fatigue. Athletic exercises should be carefully graduated and selected, so as to employ different groups of muscles, stimulating and developing each without unduly exhausting any one group. It is often forgotten, I think, that this is best accomplished by natural movements. The lower animals—for example, take a cat, in which the muscular arrangements are admirably developed—do not require to go to gymnasia for graduated athletic exercises. They run, and leap, and move in the almost unconscious enjoyment (if I may use a phrase that appears self-contradictory), of their physical organisation, and in doing so they develop each part of it. In like manner, the human being should, at least in early life, run, and leap, and play, and in more advanced life a good long daily walk will supply all that is necessary. There is a good deal of philosophy in this, as in many other common things.
- ↑ Biedermaun's fluid: chloride of sodium, five grammes; alkaline phosphate of soda, two grammes; carbonate of soda, five grammes; water, one litre.
- ↑ A little albumin is also present.
- ↑ A beautiful series of curves illustrating fatigue taken by the railway myograph was also shown by placing the glass plate in the lantern.