mental factors in fatigue. It is shown that if sufficient rests are allowed between contractions, no fatigue results. With a load of six kilograms, for instance, the flexor muscle of the finger showed no fatigue when a rest of ten seconds was given between contractions. But after complete fatigue, once the

Series of contractions of the flexor muscles of a human finger. The muscle was stimulated electrically every two seconds, and the resulting contractions were therefore involuntary. Record 1 was made when the muscle was fresh; record 2 immediately after three and one-half hours had been spent in the oral examination of students; record 3 two hours after the completion of the examination. (From Mosso's "Fatigue.")

muscles are exhausted, the utmost expenditure of will power does not enable them to contract further. A very long interval-two hours-is needed for the muscle to make a complete recovery.

3. ANOTHER FACTOR IN FATIGUE: CONSUMPTION OF

ENERGY-YIELDING SUBSTANCE

So long an interval of rest would evidently not be necessary for the removal of the poisonous metabolic products, if fatigue were due to the depressant action of these

products alone. The ergographic record, therefore, throws light upon another fundamental factor in fatigue besides the accumulation of fatigue products: the actual consumption of the material from which energy for contraction is obtained. At the termination of hard muscular work the muscle contains a lessened supply of energy-yielding material, because during contraction the processes of disassimilation or catabolism are in excess of those of assimilation or anabolism. This fundamental change in the muscle substance can be made plainer by a brief consideration of the chemical processes in contraction.

(a) THE CHEMISTRY OF MUSCULAR CONTRACTION: How GLYCOGEN IS SUPPLIED AND CONSUMED

Every voluntary muscular contraction is due to the stimulus received from the central nervous system through the nerves. Of the nature of this stimulus little is known, and the nerve elements in activity and fatigue will be considered later. We know that each muscular act has as its basis chemical processes. It is a form of combustion, as we readily recognize by the greater heat generated within us by any muscular effort. For combustion there must be union of some substance with oxygen. The union may be slow, as when iron rusts or is slowly oxidized, or fast, as when wood or coal burns with a flame. In muscular combustion the oxygen is supplied by the blood, the substance with which it combines being the so-called animal starch of the muscles, called glycogen.

Let us, then, first consider how the organism is supplied with these two essential factors for muscular action, glycogen and oxygen.

Glycogen is one of the stored materials of the muscle, a compound of carbon, hydrogen, and oxygen; and muscular tissue has the power of forming this glycogen from the sugar or dextrose brought to it by the blood. Dextrose is the form of sugar in which our carbohydrate foods (starches, sugars, etc., the bulk of our usual diet) are eventually absorbed into

the blood and carried by the blood to the muscular tissues, there to be transformed into glycogen. The stored glycogen of the muscles keeps uniting chemically with the oxygen of the blood. The glycogen is broken down into a simpler chemical form, giving off the gas carbon dioxide and other acid wastes, and releasing heat and mechanical energy in the process.

With the released energy, contraction of the muscle takes place and hence ultimately the industrial labor which is our special theme. The heat contributes to our body temperature. The chemical wastes, as we have seen, poison the whole organism unless prevented from accumulating unduly, and go to constitute what we know as fatigue.

But, as we saw above in considering the ergograph, there is another fundamental factor in fatigue which must be taken into account here: a consumption of energy-yielding material of the muscle itself. This takes place in the following manner:

Glycogen is, as it were, stored for use. It is always being replenished, always being depleted. The metabolic wastes, produced when glycogen is broken down into simpler chemical form, are constantly thrown off; the potential stuff brought by the blood is constantly being seized and built up again into living tissue. But when the muscle is active and contracts energetically, there is a run upon our glycogen. It is used up faster than it is built in muscle. The glycogen is spent so rapidly that there is not time for the bloodstream to bring back to the tissue the potential material for its repair. Glycogen may even be entirely consumed and disappear from the muscle.

But there is another organ of the body which acts further as a storehouse for glycogen. This is the liver, whose cells are so constructed that they too convert the dextrose or sugar in the blood into glycogen and retain it, until the store in the muscles is so far depleted that it must be replenished. If it were not for the stored glycogen of the liver which is supplied to the muscles at their need, starvation would more quickly end in death.

Even this provision of stored glycogen, however, does not suffice after prolonged and severe work to supply oxidizable material for muscular activity. After excessive labor the entire store of glycogen in the liver as well as in the muscle may be practically used up. Thus we have reached the other fundamental factor in fatigue,-the consumption of the energy-yielding substance itself. Not only does tissue manufacture poison for itself in its very act of living, casting off chemical wastes into the circling bloodstream; not only are these wastes poured into the blood faster with increased exertion, clogging the muscle more and more with its own noxious products; but finally, there is a depletion of the very material from which energy is obtained. The catabolic process is in excess of the anabolic. In exhaustion, the organism is forced literally to "use itself up."

We shall see later how destructive to health this phenomenon of exhaustion is, to which nervous as well as muscular tissue is subject; how long it takes to make good such losses; how exhaustion, indeed, taps the very source of our energies.

(b) How OXYGEN IS SUPPLIED FOR MUSCULAR CONTRACTION

Hitherto in this discussion we have referred constantly to the chemical reaction between glycogen and oxygen, and the results obtained when glycogen is thus broken down by oxygen. It remains now to trace how at every breath we draw, oxygen is supplied for our internal combustion of glycogen; how at every exhalation we breathe out the gas carbon dioxide-product of muscular action. The pathway for these gases is the blood.

When oxygen is breathed into the air sacs of the lungs, it comes into contact with the smallest blood vessels of the body, the capillaries. The blood in these thin-walled capillaries is separated from the oxygen in the air sacs only by moist and permeable membranes. By diffusion, the oxygen passes through these moist membranes and combines chemically with the hæmoglobin or red coloring matter of the red corpuscles in the capillaries. These tiny blood vessels, now

oxygen bearers, penetrate in a fine network to every tissue and organ in the body. As soon as the blood reaches the muscles, the loose chemical union of the haemoglobin and oxygen is again broken down, the oxygen combining with the glycogen of the muscle tissue, setting free energy, as we have seen, and evolving waste products. For, as the oxygen streams out to combine with the glycogen, there streams back in the opposite direction the gas carbon dioxide, thrown off in the chemical process.

"There is an upward rush from the lifeless to the living; a downward rush from the living to the dead."

The lifeless carbon dioxide in its turn combines with the blood, which has given its oxygen to the tissue; and in the intricate flow of our vascular system, carbon dioxide is carried back by the blood to the lungs and thence expired. We may get some notion of the combustion or chemical process carried on within our muscles by the fact that at every breath air loses about 5 per cent of its oxygen and increases in carbon dioxide a hundred fold.*

Moreover, it has been proved that after heavy muscular work, an animal gives off even larger proportions of carbon dioxide in its expired air. The physiologists Voit and Pettenkofer showed as early as 1866, that during a day in which much muscular work was done, a man expired almost twice as much carbon dioxide as during a resting day. During activity the internal combustion is more active, glycogen is being broken down more rapidly, more wastes are being thrown into the blood, more carbon dioxide is evolved. The wastes indeed accumulate more rapidly than they can be carried off, and hence, as we have seen, after excessive exer

*Notter, J. Lane, and Firth, R. H.: The Theory and Practice of Hygiene, p. 151. Third Edition. London, J. V. A. Churchill, 1908.