ELECTRO-PHYSIOLOGY 725 



boundary, or plane of demarcation, between sound and injured 

 tissue. For this reason du Bois-Reymond's current of rest is 

 called in the terminology of Hermann the ' demarcation ' current. 



The newer theories, such as Macdonald's, have sought to take 

 account of the recent developments of physical chemistry, and it is 

 unquestionable that it is here the real explanation is to be found. 

 There is little doubt that the electrical phenomena of the tissues 

 are connected with the existence in them of membranes, envelopes, 

 or sheaths, physiological if not always anatomical, which are rela- 

 tively impermeable to certain ions. When such a sheath is injured, 

 these ions, carrying with them their electrical charges, may be 

 supposed to migrate with abnormal freedom through the injured 

 part. A new distribution of electricity is thus established in the 

 tissue, and differences of potential depending upon differences 

 in the concentration of the ions at different points are set up. 

 Bernstein and Tschermak, from an investigation of the thermo- 

 dynamic relations of bio-electrical currents, have come to the con- 

 clusion that they are analogous to the currents produced by so-called 

 concentration cells i.e., arrangements o'f 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 

 ot muscle or nerve. But instead of his electromotive elements and 

 their definite arrangement, we have the ions and their definite 

 relation 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. 



Like the demarcation current, the action current and the excita- 

 tion which accompanies it may be due to changes in the permea- 

 bility 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 trans- 

 formation the electrical phenomena are the expression of chemical 

 changes or of physical (osmotic) changes, or of both, we do not 

 know. Bernstein has supposed that in the chemical process, whose 

 visible outcome is a muscular contraction, there are three stages : 

 (i) The liberation of (intra-molecular) oxygen from the molecules 

 of the living substance, and its appearance as active or atomic 



