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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and by the opening of the strong descending current, can scarcely be supposed 

 to be due to a failure of the exciting process to be developed in the nerve ; and 

 it would seem more likely that the nerve-impulse is for some reason prevented 

 from reaching the muscle which, as has been said, is the fact, the region of the 

 anode being incapable of conducting during the flow of a strong current, and 

 the region of the kathode losing its power to conduct at the instant such 

 a current is opened. 



Effect of Battery Currents upon Normal Human Nerves. In experi- 

 ments upon normal human nerves, the current cannot be applied directly to the 

 nerve, but has to be applied to the skin over the nerve. As it passes from the 

 anode, the positive electrode, through the skin, the threads of current spread 

 through the fluids and tissues beneath, somewhat as the bristles of a brush 

 spread out, and the current flows in a more or less diffuse stream toward the 

 point of exit, where the threads of current concentrate again to enter the 

 kathode, the negative electrode. This spread of the current is illustrated in 

 Figure 25. 



The density of the current entering any structure beneath the skin will 

 depend in part upon the size of the electrode directly over it that is, the 



amount to which the current is 

 concentrated at its point of en- 

 trance or exit in part on the 

 nearness of the structure to the 

 skin, and in part on the con- 

 ductivity of the tissues of the 

 organ in question as compared 

 with the tissues and fluids 

 about it. If the conditions be 

 such as are given in Figure 25, 

 the current will not, as in the 

 case of the isolated nerve, enter 

 the nerve at a given point, flow 

 longitudinally through it, and 

 then leave it at a given point ; 

 most of the threads of current 

 will pass at varying angles di- 

 agonally through the part of 

 the nerve beneath the positive 



pole, then flow through the fluids and tissues about the nerve, until, at a point 

 beneath the negative pole, the concentrating threads of current again pass 

 through the nerve. A distinction is to be drawn between the physical and 

 physiological anode and kathode. The physical anode is the extremity of the 

 positive electrode, and the physical kathode is the extremity of the negative 

 electrode; the physiological anode is the point at which the current enters the 

 tissue under consideration, and the physiological kathode is the point where it 

 leaves it. There is a physiological anode at every point where the current 



FIG. 25. Rough schema of active threads of current by 

 the ordinary application of electrodes to the skin over a 

 nerve (ulnar nerve in the upper arm). The inactive threads 

 are given in dotted lines (after Erb : Ziemsseii's Pathologic und 

 Therapie, Bd. iii. S. 76). 



