stage of fatigue proper. Decrease in working power may, in fact, be said to be the universal physical phenomenon of fatigue, whatever form of protoplasm we may be considering. Decrease in working power is accompanied by a decrease in irritability. The stimulus remaining the same, the work is diminished; but if the stimulus be increased in intensity, the protoplasm may again perform more work for a brief time. 
Fig. 2. Series of contractions of the frog's gastrocnemius muscle, excised and stimulated at intervals of two seconds. Every fiftieth contraction is recorded. The increase in the duration of the process of relaxation as fatigue proceeds is shown in the progressive lengthening of the descending limb of the curves. Sooner or later, however, all stimuli cease to be effective, and the living substance is then either exhausted or dead.
If, in our graphic record of muscular fatigue, we are employing a favorite subject of physiological study, the muscle of the frog, we observe another striking physical change. Early in the series, even before the treppe has reached its maximum, the duration of each contraction begins to increase, mainly by a slowing of the process of relaxation (Figs. 2 and 3). This may reach great proportions before exhaustion sets in. This slowing of relaxation appears to be wholly absent in the fatigue of warm-blooded, and presumably of human muscle (Fig. 4).
The fatigue of muscle tissue is thus characterized by marked physical peculiarities. It is only natural to ask what are the causes of these.
Happily it is becoming the fashion in physiology, if only slowly and following long after, it is true, the usage of John Stuart Mill, to speak less of causes than of conditions. The cause of a phenomenon is the sum total of its conditions. All conditions are causes, and it is illogical to select one or two conditions and dignify them by the seemingly superior designation. In speaking of the causes of fatigue, as is often done, one usually means its chemical conditions; for within protoplasm, when in activity, there occur certain chemical or metabolic changes, with which the phenomenon of fatigue is closely associated. These chemical changes involve two general processes, namely, the consumption of certain existing substances which are essential to the activity of the protoplasm, and the production and accumulation within it of certain waste substances. Here again the muscle has yielded us our chief knowledge. Of the substances that are consumed in protoplasmic activity, we know most about two, oxygen and carbohydrate. For all aerobic tissues or organisms a continual supply of oxygen is essential to the continuance of working power—in fact, one way of bringing on the main phenomena of fatigue seems to be by eliminating
