of heat and light—which evoke in it corresponding responses.
In the case of the contraction of muscle by mechanical or electric shock, the effect is very quick, and the contraction and relaxation take place in too short a time for detailed observation by ordinary means. Physiologists, therefore, use a contrivance by which the whole process may be recorded automatically. This consists of a lever arrangement, by which the contracting muscle writes down the history of its change, and recovery from that change. The record may be made on a travelling band of paper, which is moving at a uniform rate (fig. 62). A single response to a single stimulus consists of a contractile up-curve followed by the down-curve of recovery, the entire process being completed within a definite period of time. This autographic record gives us the most accurate information as to the characteristic properties and condition of the muscle. It gives us, too, all its individual characteristics.
Just as one wave of sound is distinguished from another by its amplitude, period, and form, so are the curves of different muscles distinguished. For example, the period for tortoise muscle may be as long as several seconds, whereas the period for the wing of an insect is as short as 1300th of a second. In the same muscle, again, the form of the curve may undergo changes from fatigue, or from the effects of various drugs. In the autographic record of the progressive death of a muscle, the graph is vigorous at first, but grows lethargic on the approach of death. In some strange way the molecules lose their mobility, rigidity supervenes, and the record of the dying muscle comes to an end. We may
