CHAPTEK IX 



ELECTRICAL EXCITATION OF NE^VE 



I. LAW OF EXCITATION BY ELECTRICAL CURRENTS 

 (DU BOIS-KEYMOND) 



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-Eeymond (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 battery 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 



