52 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Fig. 27.— 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 

 arc given in dotted lines (after Erb: Ziemsseu's Pathologie und 

 Therapie, Bd. iii. S. 76). 



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 27, 

 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 

 inters the nerve, and a physiological kathode at every point where it leaves the 

 nerve; therefore there is a physiological anode and kathode, or groups of 

 anodes and kathodes, lor the part of the nerve beneath the positive electrode, 

 and another physiological anode and kathode, or collection of anodes and 

 kathodes, for the part of the nerve beneath the negative electrode. 



To understand the effect upon the normal human nerve of opening and 

 closing the battery current, it is necessary to bear in mind three facts, viz. : 



1. At the moment that a battery current is closed, an irritating process is 

 developed at the physiological kathode, and when it is opened, at the physio- 

 logical anode. 



2. The irritating process developed at the kathode on the closing of the 

 current is stronger than that developed at the anode on the opening of the 

 current. 



3. The effect of the current is greatest where its density is greatest. 



The amount of the irritation process developed in a motor nerve is esti- 

 mated from the amount of the contraction of the muscle. The contraction 



