THEORIES OF DEMARCATION AND ACTION CURRENTS 829 



tion cells i.e., arrangements of solutions of electrolytes of different 

 concentration in contact with each other. Since the development of 

 the new electrical condition depends upon the fundamental structure 

 of the tissue, these modern views lead us back to du Bois-Reymond's 

 doctrine of a pre-existing electrical equilibrium connected with the 

 essential physiological properties of muscle or nerve. But instead of 

 his electromotive elements and their definite arrangement, we have 

 the ions and their definite relaticn to the semi-permeable membranes. 



Relation between the Action Current and Functional Activity. 

 Although the negative variation is so general an accompaniment of 

 excitation, and is even within tolerably wide limits, in muscle and nerve 

 at least, pretty nearly proportional to the strength of the stimulus, it 

 is at present impossible to say definitely what the chemical or physical 

 changes are which underlie it. Unquestionably the electrical changes 

 are closely related to the excitatory process and to the functional 

 activity of the tissues. In the case of nerve some writers, indeed, 

 assume that the redistribution of potential associated with the excited 

 state is identical with the nervous impulse, but the common view is 

 that the negative variation is an accompaniment of some other change 

 which constitutes the propagated disturbance. There is at present no 

 clear experimental evidence sufficient to decide the question. From 

 time to time attempts have been made to show that the two processes 

 can be dissociated, but none of the experiments so far reported are 

 really crucial. 



Like the demarcation current, the action current and the excitation 

 which accompanies it may be due to changes in the permeability of 

 membranes or changes in the concentration of certain ions. 



Although the electromotive changes caused by excitation are much 

 more transient than those caused by injury, everything suggests that 

 there must be some deep analogy between the two conditions. Some 

 have supposed that what may be called a subdued and more or less 

 permanent excitation exists in the neighbourhood of the injured tissue, 

 an excitation which, like some other forms, does not spread, and that 

 this explains the similarity of electrical condition in activity and injury. 



It is, of course, clear that energy must be transformed to produce an 

 electromotive force capable of doing work. It may be assumed that 

 this energy is ultimately derived from the stock of chemical energy in 

 the tissue-substance. But whether in the final transformation the 

 electrical phenomena are the expression of chemical changes or of 

 physical (osmotic) changes, or of both, we do not know. In the case of 

 muscle it is possible that the liberation cf lactic acid, which there are 

 several reasons for regarding as essentially concerned in the initiation 

 of the mechanical change, is associated in some way with the appearance 

 of the negative variation. It is known that the latter, although it 

 begins before the contraction, and very rapidly reaches its maximum, 

 declines more gradually, so that it overlaps the mechanical change of 

 form. This is particularly well seen in veratrinized muscles (p. 755), in 

 which the electrical variation, like the contraction, is greatly prolonged 

 (Garten) . 



Polarization of Muscle and Nerve. We have already spoken of 

 electrical excitation and of the changes of excitability caused by 

 the passage of a constant current (p. 785). We are now to see that 

 these physiological effects are accompanied by, and indeed very 

 closely related to, more physical changes which the galvanometer 

 or electrometer reveals to us. Since these throw light on the 



