e-curve: period,
amplitude, form--Modification of response-curves.

One of the most striking effects of external disturbance on certain
types of living substance is a visible change of form. Thus, a piece of
muscle when pinched contracts. The external disturbance which produced
this change is called the stimulus. The body which is thus capable of
responding is said to be irritable or excitable. A stimulus thus
produces a state of excitability which may sometimes be expressed by
change of form.

#Mechanical response to different kinds of stimuli.#--This reaction under
stimulus is seen even in the lowest organisms; in some of the amoeboid
rhizopods, for instance. These lumpy protoplasmic bodies, usually
elongated while creeping, if mechanically jarred, contract into a
spherical form. If, instead of mechanical disturbance, we apply salt
solution, they again contract, in the same way as before. Similar
effects are produced by sudden illumination, or by rise of temperature,
or by electric shock. A living substance may thus be put into an
excitatory state by either mechanical, chemical, thermal, electrical, or
light stimulus. Not only does the point stimulated show the effect of
stimulus, but that effect may sometimes be conducted even to a
considerable distance. This power of conducting stimulus, though common
to all living substances, is present in very different degrees. While in
some forms of animal tissue irritation spreads, at a very slow rate,
only to points in close neighbourhood, in other forms, as for example in
nerves, conduction is very rapid and reaches far.

The visible mode of response by change of form may perhaps be best
studied in a piece of muscle. When this is pinched, or an electrical
shock is sent through it, it becomes shorter and broader. A responsive
twitch is thus produced. The excitatory state then disappears, and the
muscle is seen to relax into its normal form.

#Mechanical lever recorder.#--In the case of contraction of muscle, the
effect is very quick, the twitch takes place in too short a time for
detailed observation by ordinary means. A myographic apparatus is
therefore used, by means of which the changes in the muscle are
self-recorded. Thus we obtain a history of its change and recovery from
the change. The muscle is connected to one end of a writing lever. When
the muscle contracts, the tracing point is pulled up in one direction,
say to the right. The extent of this pull depends