154 



THE PHYSIOLOGY OF NERVE 



The effects of unipolar stimulation, however, are complicated by 

 the fact that the electrodes cannot be applied directly to the nerve 

 but only to the skin overlying, and hence, a number of peripolar re- 

 gions invariably develop around the actual poles upon the nerve. If 

 the stimulating electrode is anodic and the indifferent electrode ca- 

 thodic, the various electrical lines enter the tissues as through a nar- 

 row gate and then spread out fan-like underneath, constantly seeking 

 paths of least resistance. At the cathodic pole, these lines again con- 

 verge and are finally combined into a number of smaller bundles. At 

 every point where these lines enter the nerve there is established a 

 secondary anode, and wherever they emerge, a secondary cathode. 

 In this way, a number of secondary or physiological anodes and ca- 



FiG. 92. — Rough Schema op Actitb Threads of Current by the Ordinary 



Application of Electrodes to the Skin over a Nerve (Ulnar Nerve). 



The inactive threads are given in dotted lines (after Erb) . 



thodes are developed beneath the primary or physical anode as well as 

 below the physical cathode. In brief, therefore, it may be stated that 

 the results of the stimulation of human muscle and nerve are depen- 

 dent upon the interaction of these physical and physiological poles. 



A fuller explanation must take into account that the contraction, 

 following the making of the current, is developed at the physiological 

 cathode, while the one following the breaking of the current, is devel- 

 oped at the physiological anode. Now, since the stimulating electrode 

 may be made either anodic or cathodic, and since physiological anodes 

 and cathodes are developed in either case, four possibilities arise, 

 namely : 



(1) Anodic Stimulation : 



(a) On making we obtain the so-called anodic closing contraction in 

 consequence of excitatory changes resulting at the physiological 

 cathode beneath the physical anode. 



