the constructive processes are in abeyance, and
there is no active circulation for the removal of fatigue products.

It has been said before that the E.M. variation caused by stimulus is
the concomitant of a disturbance of the molecules of the responsive
tissues from their normal equilibrium, and that the curve of recovery
exhibits the restoration of the tissue to equilibrium.

#No fatigue when sufficient interval between successive stimuli.#--We may
thus gather from a study of the response-curve some indication of the
molecular distortion experienced by the excited tissue. Let us first
take the case of an experiment whose record is given in fig. 20, _a_.
It will be seen from that curve that one minute after the application of
stimulus there is a complete recovery of the tissue; the molecular
condition is exactly the same at the end of recovery as in the beginning
of stimulation. The second and succeeding response-curves therefore are
exactly similar to the first, _provided a sufficient interval has been
allowed in each case for complete recovery_. There is, in such a case,
no diminution in intensity of response, that is to say, no fatigue.

We have an exactly parallel case in muscles. _'In muscle with normal
circulation and nutrition there is always an interval between each pair
of stimuli, in which the height of twitch does not diminish even after
protracted excitation, and no fatigue appears.'_[10]

[Illustration: FIG. 20.--RECORD SHOWING DIMINUTION OF RESPONSE WHEN
SUFFICIENT TIME IS NOT ALLOWED FOR FULL RECOVERY
In (_a_) stimuli were applied at intervals of one minute; in (_b_) the
intervals were reduced to half a minute; this caused a diminution of
response. In (_c_) the original rhythm is restored, and the response
is found to be enhanced. (Radish.)]

#Apparent fatigue when stimulation frequency increased.#--If the rhythm of
stimulation frequency be now changed, and made quicker, certain
remarkable modifications will appear in the response-curves. In fig. 20,
the first part shows the responses at one minute interval, by which time
the individual recovery was complete.

The rhythm was now changed to intervals of half a minute, instead of
one, while the stimuli were maintained at the same intensity as before.
It will be noticed (fig. 20, _b_) that these responses appear much
feebler than the first set, in spite of the equality of stimulus. An
inspection of the figure may perhaps throw some lig