In the muscular curve we notice (1) the rapidity of its rise, which indicates the rapidity of contraction of the muscle; (2) its length, indicating the duration of contraction; (3) its height, indicating power of contraction; and (4) slowness of fall, indicating the condition of extensibility.

The muscular contraction is modified by numerous conditions.

One of these is the strength of stimulus.

The stimulus usually applied is electricity, as its strength can be more easily regulated, and it does not destroy the muscle so readily as mechanical or chemical irritants.

With a weak current, making (closing) has no action on the muscle, but breaking (opening) causes contraction.

a

a

b

b

c

c

d

d

Fig. 40. - Muscle curves, showing the different appearances they present according to the rate at which the recording cylinder revolves. a is a curve with a very slowly revolving cylinder; b, c, and d are curves with increasing speed of rotation. c is written with a lever pointing in the opposite direction from that with which a and b are recorded, and the curve therefore inclines to the other side.

A moderate current gives contraction both in making and breaking, but that of making is comparatively small (Fig. 41). With a strong current no difference is observed.

Fig. 41.   Shows effect of making and breaking shocks. These are normal muscle curves with a still quicker rotating cylinder than in Fig. 4od. The first is caused by irritating the muscle by making (closing) a constant current, and the second by breaking (opening) it.

Fig. 41. - Shows effect of making and breaking shocks. These are normal muscle curves with a still quicker rotating cylinder than in Fig. 4od. The first is caused by irritating the muscle by making (closing) a constant current, and the second by breaking (opening) it.

The more intense the stimulus, the higher and longer is the curve. The increase in height is shown in Fig. 42.

Fig. 42. Tracing of the contractions of a muscle with stimuli of varying strength. The numbers indicate the distance in centimetres of the secondary from the primary coil in the induction apparatus.

Fig. 42.-Tracing of the contractions of a muscle with stimuli of varying strength. The numbers indicate the distance in centimetres of the secondary from the primary coil in the induction apparatus. As and Des indicate the ascending and descending direction of the current.

Cold renders contraction slower, lower, and more prolonged (Fig. 43 b).

Heat renders it quicker, higher, and shorter (Fig. 43 a).