on the amount of
contraction. A band of paper or a revolving drum-surface moves at a
uniform speed at right angles to the direction of motion of the writing
lever. When the muscle recovers from the stimulus, it relaxes into its
original form, and the writing point traces the recovery as it moves now
to the left, regaining its first position. A curve is thus described,
the rising portion of which is due to contraction, and the falling
portion to relaxation or recovery. The ordinate of the curve represents
the intensity of response, and the abscissa the time (fig. 1).

[Illustration: FIG. 1.--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.]

#Characteristics of the response-curve: (1) Period, (2) Amplitude, (3)
Form.#--Just as a wave of sound is characterised by its (1) period, (2)
amplitude, and (3) form, so may these response-curves be distinguished
from each other. As regards the period, there is an enormous variation,
corresponding to the functional activity of the muscle. For instance, in
tortoise it may be as high as a second, whereas in the wing-muscles of
many insects it is as small as 1/300 part of a second. 'It is probable
that a continuous graduated scale might, as suggested by Hermann, be
drawn up in the animal kingdom, from the excessively rapid contraction
of insects to those of tortoises and hibernating dormice.'[1]
Differences in form and amplitude of curve are well illustrated by
various muscles of the tortoise. The curve for the muscle of the neck,
used for rapid withdrawal of the head on approach of danger, is quite
different from that of the pectoral muscle of the same animal, used for
its sluggish movements.

Again, progressive changes in the same muscle are well seen in the
modifications of form which consecutive muscle-curves gradually undergo.
In a dying muscle, for example, the amplitude of succeeding curves is
continuously diminished, and the curves themselves are elongated.
Numerous illustrations will be seen later, of the effect, in changing
the form of the curve, of the increased excitation or depre