1912.] The Process of Excitation in Nerve and Muscle. 505 



founded their original statement, discovered a small trace of a second electric 

 response in the cooled region of the nerve. This response was greatly delayed 

 and might easily have been regarded as a part of the single response. Never- 

 theless its presence limited the possible inference from the experiment to 

 the statement that an effective propagated disturbance might be accompanied 

 by a scarcely measurable disturbance of electric potential. In the same paper 

 Gotch brought the further evidence that the neighbourhood of a recent injury 

 may be incapable of producing an electric response of sufficient magnitude to 

 affect 'a sensitive capillary electrometer, though it is a region obviously able 

 to propagate a disturbance, since quite weak stimuli applied to it will provoke 

 a disturbance which is propagated to other parts of the nerve. This observa- 

 tion has not, as far as I am aware, been called in question, and we must admit 

 at least that the nervous impulse may be accompanied by an electric response 

 so small as to be imperceptible by the capillary electrometer. 



Have we here a satisfying proof of want of parallelism between the propa- 

 gated disturbance and the electric response ? In our interpretation of the 

 experiment we must bear in mind the limitations which are imposed upon our 

 measurement of both phenomena, and must find a want of parallelism only 

 if we are clear that whereas the one has undergone very great diminution the 

 other has not. In measuring the electromotive force of the electric response 

 we are at a great disadvantage. It is an old observation, which has lately 

 been brought into prominence,* that the electromotive force which we measure 

 with our electrometers is only an unknown fraction of that actually present 

 in the tissue. We are leading off the fall of potential between two points of 

 that external part of the circuit in a nerve or muscle which is made up of the 

 lymph or other fluid retained between the fibres. What part of the total 

 resistance of the circuit that represents, and consequently what part of the 

 whole potential difference is led off, we do not know. Still more are we 

 unable to obtain a relative measure of the size of a propagated disturbance at 

 any given point in a tissue. Our one method is in practice to measure the 

 consequence of the propagated disturbance— either as muscular contraction 

 or as electric response — after it has emerged from a region where it has been 

 modified. If the experiments of Adrian, of which I have already spoken, 

 prove to be valid, such methods can in reality tell us only whether a 

 disturbance has or has not been extinguished. Consider, for example, such 

 an experiment as that of Gotch and Burch on locally cooled nerve. Within 

 the cooled region the second electric response was scarcely perceptible ; in 

 the warmer region beyond it was large. There seems no valid argument 

 against the supposition that the second propagated disturbance was also 

 * Cf. Bernstein, 'Arch. f. d. ges. Physiol.,' 1902, vol. 92, p. 562. 



