CHAPTEE IX 



ELECTRICAL EXCITATION OF NERVE 



I. LAW OF EXCITATION BY ELECTRICAL CURRENTS 

 (DU BOIS-REYMOND) 



THE electrical current again ranks first among all the artificial 

 means of nerve-excitation. A succession of experiments in this 

 direction dates from very early days, and forms one of the most 

 interesting chapters in physiology. 



Du Bois - Key mond (1) has given an admirable historical 

 survey of this part of the subject. At its outset we encounter 

 the fact that a, motor nerve like the corresponding striated muscle, 

 but in a still higher degree is apparently excited only at the 

 moment of closing, or opening, a IxMcry current. In fact, du Bois' 

 " general law of electrical excitation of nerve " was at first laid 

 down for indirect excitation of the muscle, and was only extended 

 at a later period to direct muscular stimulation. The law in 

 its original form ran as follows : 



" It is not the absolute value of current-density in the nerve, 

 at any given moment, that determines the response of the muscle, 

 but the variations of this value from moment to moment : the 

 stimulus to movement consequent on these changes being the 

 more considerable according as they are (in a given interval) 

 greater in magnitude, or more rapid in their onset." 



If a motor nerve, still attached to the muscle, is laid across 

 unpolarisable electrodes, and excited by the closure or opening of 

 a sufficiently strong battery current, a single rapid twitch of the 

 muscle appears at make and often at break also, after which it 

 returns to the normal resting position. The most careful observa- 

 tion fails to detect any permanent shortening during closure, or 



