normal one minute before, to
five minutes after, the application of the reagent (fig. 93, _a_). In
another record the arrest is more pronounced, i.e. there is now no
recovery (fig. 93, _b_). Note also that the maximum is attained much
later. Stimuli applied after the arrest produce no effect, as if the
molecular mechanism became, as it were, clogged or locked up.

In connection with this it is interesting to note that the effect of
veratrine poison on muscle is somewhat similar. This reagent not only
diminishes the excitability, but causes a very great prolongation of the
period of recovery.

In connection with the action of chemical reagents the following points
are noteworthy.

[Illustration: FIG. 93.--'MOLECULAR ARREST' BY THE ACTION OF 'POISON'
In each, curves to the left show the normal response, curve to the right
shows the effect of poison. In (_a_) the arrest is evidenced by
prolongation of period of recovery. In (_b_) there is no recovery.]

(1) The effect of these reagents is not only to increase or diminish the
height of the response-curve, but also to modify the time relations. By
the action of some the latent period is diminished, others produce a
prolongation of the period of recovery. Some curious effects produced by
the change of time relations have been noticed in the account given of
diphasic variation (see p. 113).

(2) The effect produced by a chemical reagent depends to some extent on
the previous condition of the wire.

(3) A certain time is required for the full development of the effect.
With some reagents the full effect takes place almost instantaneously,
while with others the effect takes place slowly. Again the effect may
with time reach a maximum, after which there may be a slight decline.

(4) The after-effects of the reagents may be transitory or persistent;
that is to say, in some cases the removal of the reagent causes the
responses to revert to the normal, while in others the effect persists
even after the removal of all traces of the reagent.

#Opposite effects of large and small doses.#--There remains a very curious
phenomenon, known not only to students of physiological response but
also known in medical practice, namely that of the opposite effects
produced by the same reagent when given in large or in small doses.
Here, too, we have the same phenomena reproduced in an extraordinary
manner in inorganic response. The same reagent which becomes a 'poison'
in large qu