examine it first in the tissue of voluntary muscle, which affords certain advantages for study over other tissues, because of the ease of employing the graphic method and other physical, as well as chemical, methods. If, shortly after the death of an animal, a single muscle, such as a muscle of a leg, be removed from the body, be attached to the usual muscle lever of the physiological laboratory, and be stimulated, in the usual manner at regular intervals, beginning when the muscle is fresh and continuing until it is well fatigued, the graphic record of the
Fig. 1. Series of contractions of a frog's sartorius muscle, excised and stimulated at intervals of two and one half seconds. Each successive vertical line is the record of a single contraction. The contractions at first increase in extent, this stage constituting the treppe, and later decrease, this stage constituting fatigue.
series of resulting contractions presents a striking picture. Both the extent and the duration of the contractions may be affected. There appears early an increase in the extent of the contractions, which proceeds gradually to a maximum. This is shown in the graphic record as an increase in the height of the successive muscle curves (Fig. 1) and has been called, not inappropriately, the staircase, or treppe. The treppe signifies that in the early stages of muscular activity the working power of the muscle is progressively augmented—there is a temporary improvement in the power to work. This in turn means that what physiologists call the irritability of the muscle, or, in other words, its power of responding to a stimulus, has become greater; hence the same stimulus is followed by a greater contraction. A progressive improvement in the power to work in the early portion of a task, I may say, is not peculiar to muscle. We all must have noticed it in our own experience, with both physical and mental labor. It has also been demonstrated by laboratory methods in nerves, the central nervous system, and other animal and plant tissues; and it is probably a characteristic of all living substance. An analogous phenomenon is observed when living substance is put under the influence of certain drugs—a small quantity of alcohol, for example, often effects a temporary improvement in the individual's power of performing work.
Following the treppe, the muscle may perform maximal contractions for a considerable time; it is in its best working condition; its irritability is such that a given stimulus calls forth the greatest contraction of which it is capable. But sooner or later the contractions begin to diminish in extent; they sink to the level of the original amount and below it; the muscle becomes gradually weaker and weaker, until, with long-continued effort, it may finally cease altogether to lift the weight. This decrease in working power from the maximum characterizes the
