Page:Collected Physical Papers.djvu/281

thus find out the effects of various external influences by studying the changes in the muscle-curve.

Fig. 62. Mechanical Lever Recorder. The muscle M with the attached bone is securely held at one end, the other end being connected with the writing lever. Under the action of stimulus the contracting muscle pulls the lever and moves the tracing point to the right over the travelling recording surface P. When the muscle recovers from contraction, the tracing point returns to its original position. See on P the record of muscle curve.

We may stimulate the living substance in various ways—by light, or by thermal, chemical, electrical, or mechanical stimuli. Of these, the electric means of stimulation is the most convenient, whereas the mechanical causes fewest complications. With regard to the response of living substances, the most important matters for study are the responses to single stimulus and the modification of response by fatigue and by drugs.

A single shock causes a twitch, but the muscle soon recovers its original form. The rising portion of the curve is due to contraction, whereas the falling portion exhibits recovery (see curve in fig. 62).