tigue-products, while recovery is due to assimilation, for which
material is brought by the blood-supply--has long been seen to be
inadequate, since the restorative effect succeeds a short period of rest
even in excised bloodless muscle. But that the phenomena of fatigue and
recovery were not primarily dependent on dissimilation or assimilation
becomes self-evident when we find exactly similar effects produced not
only in plants, but also in metals (fig. 113). It has been shown, on the
other hand, that these effects are primarily due to cumulative residual
strains, and that a brief period of rest, by removing the overstrain,
removes also the sign of fatigue.

#Staircase effect.#--The theory of dissimilation due to stimulus reducing
the functional activity below par, and thus causing fatigue, is directly
negatived by what is known as the 'staircase' effect, where successive
equal stimuli produce increasing response. We saw an exactly similar
phenomenon in plants and metals, where successive responses to equal
stimuli exhibited an increase, apparently by a gradual removal of
molecular sluggishness (fig. 114).

[Illustration: FIG. 114.--'STAIRCASE' IN MUSCLE, PLANT, AND METAL]

[Illustration: FIG. 115.--INCREASED RESPONSE AFTER CONTINUOUS
STIMULATION IN NERVE AND METAL
The normal response in animal tissue is represented 'down,' in metal
'up.']

#Increased response after continuous stimulation.#--An effect somewhat
similar, that is to say, an increased response, due to increased
molecular mobility, is also shown sometimes after continuous
stimulation, not only in animal tissues, but also in metals (fig. 115).

#Modified response.#--In the case of nerve we saw that the normal
response, which is negative, sometimes becomes reversed in sign, i.e.
positive, when the specimen is stale. In retina again the normal
positive response is converted into negative under the same conditions.
Similarly, we found that a plant when withering often shows a positive
instead of the usual negative response (fig. 28). On nearing the
death-point, also by subjection to extremes of temperature, the same
reversal of response is occasionally observed in plants. This reversal
of response due to peculiar molecular modification was also seen in
metals.

[Illustration: FIG. 116.--MODIFIED ABNORMAL RESPONSE IN (A) NERVE AND
(M) METAL CONVERTED INTO NORMAL, AFTER CONTINUOUS STIMULATION
(A) is the record for nerve (recording galvanometer