ssion
produced by various agencies.

Thus these response records give us a means of studying the effect of
stimulus, and the modification of response, under varying external
conditions, advantage being taken of the mechanical contraction produced
in the tissue by the stimulus. But there are other kinds of tissue where
the excitation produced by stimulus is not exhibited in a visible form.
In order to study these we have to use an altogether independent method,
the method of electric response.

FOOTNOTES:

[1] Biedermann, _Electro-physiology_, p. 59.

CHAPTER II

ELECTRIC RESPONSE

Conditions for obtaining electric response--Method of injury--Current of
injury--Injured end, cuproid: uninjured, zincoid--Current of
response in nerve from more excited to less excited--Difficulties of
present nomenclature--Electric recorder--Two types of response,
positive and negative--Universal applicability of electric mode of
response--Electric response a measure of physiological
activity--Electric response in plants.

Unlike muscle, a length of nerve, when mechanically or electrically
excited, does not undergo any visible change. That it is thrown into an
excitatory state, and that it conducts the excitatory disturbance, is
shown however by the contraction produced in an attached piece of
muscle, which serves as an indicator.

But the excitatory effect produced in the nerve by stimulus can also be
detected by an electrical method. If an isolated piece of nerve be taken
and two contacts be made on its surface by means of non-polarisable
electrodes at A and B, connection being made with a galvanometer, no
current will be observed, as both A and B are in the same
physico-chemical condition. The two points, that is to say, are
iso-electric.

If now the nerve be excited by stimulus, similar disturbances will be
evoked at both A and B. If, further, these disturbances, reaching A and
B almost simultaneously, cause any electrical change, then, similar
changes taking place at both points, and there being thus no relative
difference between the two, the galvanometer will still indicate no
current. This null-effect is due to the balancing action of B as against
A. (See fig. 2, _a_.)

#Conditions for obtaining electric response.#--If then we wish to detect
the response by means of the galvanometer, one means of doing so will
lie in the abolition of this balance, which may be accomplished by
making one