metimes even in nerve, which otherwise, generally
speaking, gives uniform responses. Of this effect, no satisfactory
theory has as yet been offered. It is in direct contradiction to that
theory which supposes that each stimulus is followed by dissimilation or
break-down of the tissue, reducing its function below par. For in these
cases the supposed dissimilation is followed not by a decrease but by an
increase of functional activity. This 'staircase effect' I have shown to
be occasionally exhibited by plants. I have also found it in metals. In
the last chapter we have seen that a wire often falls, especially after
resting for a long time, into a state of comparative sluggishness, and
that this molecular inertness then gradually gives place to increased
mobility under stimulation. As a consequence, an increased response is
thus obtained. I give in fig. 74, _b_, a series of responses to uniform
stimuli, exhibited by platinum which had been at rest for some time.
This effect is very clearly shown here. So we see that in a substance
which has previously been in a sluggish condition, stimulation confers
increased mobility. Response thus reaches a maximum, but continued
stimulation may afterwards produce overstrain, and the subsequent
responses may then show a decline. This consideration will explain
certain types of responses exhibited by muscles, where the first part
of the series exhibits a staircase increase followed by declining
responses of fatigue.

[Illustration: FIG. 74.--'STAIRCASE' EFFECT
(_a_) in muscle (after Engelmann).
(_b_) in metal.]

#Reversed response due to molecular modification and its transformation
into normal after continuous stimulation (1) in nerve.#--Reference has
already been made to the fact that a nerve which, when fresh, exhibited
the normal negative response, will often, if kept for some time in
preservative saline, undergo a molecular modification, after which it
gives a positive variation. Thus while the response given by fresh nerve
is _normal_ or negative, a stale nerve gives _modified_, i.e. reversed
or positive, response. This peculiar modification does not always occur,
yet is too frequent to be considered abnormal. Again, when such a nerve
is subjected to tetanisation or continuous stimulation, this modified
response tends once more to become normal.

It is found that not only tetanisation, but also CO_2 has the power of
converting the modified response into normal. Hence it has bee