INORGANIC RESPONSE--EFFECT OF CHEMICAL REAGENT
Action of chemical reagents--Action of stimulants on metals--Action of
depressants on metals--Effect of 'poisons' on metals--Opposite
effect of large and small doses.
We have seen that the ultimate criterion of the physiological character
of electric response is held to be its abolition when the substance is
subjected to those chemical reagents which act as poisons.
[Illustration: FIG. 88.--ACTION OF POISON IN ABOLISHING RESPONSE IN
NERVE (WALLER)]
#Action of chemical reagents.#--Of these reagents, some are universal in
their action, amongst which strong solutions of acids and alkalis, and
salts like mercuric chloride, may be cited. These act as powerful toxic
agents, killing the living tissue, and causing electric response to
disappear. (See fig. 88.) It must, however, be remembered that there are
again specific poisons which may affect one kind of tissue and not
others. Poisons in general may be regarded as extreme cases of
depressants. As an example of those which produce moderate physiological
depression, potassium bromide may be mentioned, and this also diminishes
electric response. There are other chemical reagents, on the other hand,
which produce the opposite effect of increasing the excitability and
causing a corresponding exaltation of electric response.
We shall now proceed to inquire whether the response of inorganic bodies
is affected by chemical reagents, so that their excitability is exalted
by some, and depressed or abolished by others. Should it prove to be so,
the last test will have been fulfilled, and that parallelism which has
been already demonstrated throughout a wide range of phenomena, between
the electric response of animal tissues on the one hand, and that of
plants and metals on the other, will be completely established.
#Action of stimulants on metals.#--We shall first study the stimulating
action of various chemical reagents. The method of procedure is to take
a series of normal responses to uniform stimuli, the electrolyte being
water. The chemical reagent whose effect is to be observed is now added
in small quantity to the water in the cell, and a second series of
responses taken, using the same stimulus as before. Generally speaking,
the influence of the reagent is manifested in a short period, but there
may be occasional instances where the effect takes some time to develop
fully. We must remember that by the introduction of the chemical reagent
some change may be produced in the internal resistance of the cell. The
effect of this on the deflection is eliminated by interposing a very
high external resistance (from one to five megohms) in comparison with
which the internal resistance of the cell is negligible. The fact that
the introduction of the reagent did not produce any variation in the
total resistance of the circuit was demonstrated by taking two
deflections, due to a definite fraction of a volt, before and after the
introduction of the reagent. These deflections were found equal.
[Illustration: FIG. 89.--STIMULATING ACTION OF Na_2CO_3 ON TIN]
I first give a record of the stimulating action of sodium carbonate on
tin, which will become evident by a comparison of the responses before
and after the introduction of Na_2CO_3 (fig. 89). The next record
shows the effect of the same reagent on platinum (fig. 90).
[Illustration: FIG. 90.--STIMULATING ACTION OF Na_2CO_3 ON PLATINUM]
#Action of depressants.#--Certain other reagents, again, produce an
opposite effect. That is to say, they diminish the intensity of
response. The record given on the next page (fig. 91) shows the
depressing action of 10 per cent. solution of KBr on tin.
[Illustration: FIG. 91.--DEPRESSING EFFECT OF KBR (10 PER CENT.) ON THE
RESPONSE OF TIN]
#Effect of 'poison.'#--Living tissues are killed, and their electric
responses are at the same time abolished by the action of poisons. It is
very curious that various chemical reagents are similarly effective in
killing the response of metals. I give below a record (fig. 92) to show
how oxalic acid abolishes the response. The depressive effect of this
reagent is so great that a strength of one part in 10,000 is often
sufficient to produce complete abolition. Another notable point with
reference to the action of this reagent is the persistence of
after-effect. This will be clearly seen from an account of the following
experiment. The two wires A and B, in the cell filled with water, were
found to give equal responses. The wires were now lifted off, and one
wire B was touched with dilute oxalic acid. All traces of acid were next
removed by rubbing the wire with cloth under a stream of water. On
replacing the wire in the cell, A gave the usual response, whereas that
of B was found to be abolished. The depression produced is so great and
passes in so deep that I have often failed to revive the response, even
after rubbing the wire with emery paper, by which the molecular layer on
the surface must have been removed.
[Illustration: FIG. 92.--ABOLITION OF RESPONSE BY OXALIC ACID]
We have seen in the molecular model (fig. 62, _d_, _e_) how the
attainment of maximum is delayed, the response diminished, and the
recovery prolonged or arrested by increase of friction or reduction of
molecular mobility.
It would appear as if the reagents which act as poisons produced some
kind of molecular arrest. The following records seen to lend support to
this view. If the oxalic acid is applied in large quantities, the
abolition of response is complete. But on carefully adding just the
proper amount I find that the first stimulus evokes a responsive
electric twitch, which is less than the normal, and the period of
recovery is very much prolonged from the 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 quantities may act as a stimulant when applied in small doses.
This is seen in record fig. 94, in which (_a_) gives the normal
responses in water; KHO solution was now added so as to make the
strength three parts in 1,000, and (_b_) shows the consequent
enhancement of response. A further quantity of KHO was added so as to
increase the strength to three parts in 100. This caused a complete
abolition (_c_) of response.
[Illustration: FIG. 94.--OPPOSITE EFFECTS OF SMALL AND LARGE DOSES (TIN)
(_a_) is the normal response; (_b_) is the stimulating action of small
dose of potash (3 parts in 1,000); (_c_) is the abolition of
response with a stronger dose (3 parts in 100).]
It will thus be seen that as in the case of animal tissues and of
plants, so also in metals, the electrical responses are exalted by the
action of stimulants, lowered by depressants, and completely abolished
by certain other reagents. The parallelism will thus be found complete
in every detail between the phenomena of response in the organic and the
inorganic.