966 



SCIENCE 



[N. S. Vol. XXXVII. No. 965 



that the surface-films or plasma-membranes 

 of the irritable elements play any such spe- 

 cial part in stimulation. 



There are in fact many indications that 

 this is the ease. Investigation of the condi- 

 tions of electrical excitation — undertaken 

 quite without reference to the problem we 

 are considering — has shown that the semi- 

 permeable membranes of irritable tissues 

 are intimately concerned in stimulation. 

 The first definite proof of this was brought 

 forward in 1899 by Nernst. He was 

 struck with the fact that Tesla currents 

 (or alternating currents of high frequency) 

 may be passed throiigh irritable tissues (or 

 through the human body) without causing 

 stimulation; while if the frequency of the 

 current is sufficiently lowered, but without 

 altering its intensity, strong stimulation re- 

 sults. Now what does this mean? Evi- 

 dently that the current must flow for a cer- 

 tain minimal time in a constant direction 

 in order to stimulate. Mere conduction of 

 a given quantity of electricity through an 

 irritable tissue is not in itself sufficient to 

 cause stimulation. There is some kind of 

 cumulative effect depending on a steady 

 flow in one direction. What physical pecu- 

 liarities of the living tissue condition this 

 remarkable peculiarity? Nernst pointed 

 out that a living tissue in its relation to the 

 electric current is an electrolytic conductor, 

 which, however, is not homogeneous like an 

 ordinary salt-solution, but peculiar in being 

 sub-divided at intervals by semi-permeable 

 partitions, the cell-membranes. When 

 therefore the current starts to flow it car- 

 ries as usual anions toward the anode and 

 cations toward the cathode, but at the semi- 

 permeable membranes this movement is 

 blocked; the concentration of anions thus 

 tends to rise above that of cations on the 

 side of the membrane facing the cathode 

 and vice versa, and the above behavior of 

 the tissue may be partly explained if we 



assume that these changes of concentration 

 must reach a certain degree if stimulation 

 is to result. For this effect time is re- 

 quired. Hence, if the current is reversed 

 too soon, the stimulating effect is annulled, 

 and the tissue remains unaffected. Now, if 

 the assumption is true that stimulation is 

 the expression of a change of electrical 

 polarization, due to a change in the con- 

 centration of ions at the membranes, the 

 time during which the current must flow in 

 order to produce a given polarization-effect 

 ought to correspond with that needed for a 

 given stimulation-effect. Nernst 's analy- 

 sis shows that the polarizing action varies 

 directly with the intensity of the current 

 (i. e., the quantity of electricity — i. e, ions 

 — transported in unit time) and with the 

 square root of its duration (S^Kiy/t), 

 and observations on a variety of irritable 

 tissues have shown that the stimulating ac- 

 tion of a given current does vary in essen- 

 tially this manner with the duration of its 

 flow in one direction {i. e., inversely with 

 the square root of the number of alterna- 

 tions in the case of an alternating current) . 

 It seems clear then that electrical stimula- 

 tion is dependent on the polarization- 

 changes produced by the current at the 

 semi-permeable membranes of the irritable 

 elements. 



One main result of these investigations is 

 thus to localize the stimulating action of 

 the current at the semi-permeable mem- 

 branes, and to indicate that a change in the 

 electrical polarization of these membranes 

 is an essential feature of stimulation. It is 

 evident that this result does not constitute 

 a complete analysis of the nature of stimu- 

 lation. But it indicates that some change 

 in the membrane is essential to this process. 

 An electrical variation accompanies every 

 normal stimulation and undoubtedly forms 

 an inseparable feature of the process, but 

 its conditions are still imperfectly under- 



