ACTION OF ELECTRICITY ON THE NERVES 483 



it is evident that the velocity of the nerve-current must vary very con- 

 siderably in different individuals. 



Action of Electricity on the Nerves. So long as the nerves retain 

 their excitability and anatomical integrity, they will respond to properly 

 applied electric stimulus. Experiments may be made on the exposed 

 nerves in living animals or in animals just killed ; and of all classes, the 

 cold-blooded animals present the most favorable conditions, on account 

 of the persistence of nervous and muscular excitability for a consider- 

 able time after death. Experimenters commonly use frogs, on account 

 of the long persistence of the physiological properties of their tissues 

 and the facility with which certain parts of the nervous system can be 

 exposed. For ordinary experiments on nervous conduction, the parts 

 are prepared by detaching the posterior extremities, removing the skin, 

 and cutting away the bone and muscles of the thigh so as to leave the 

 leg with the sciatic nerve attached. A frog's leg thus isolated presents 

 a nervous trunk one or two inches (25 or 50 millimeters) in length, 

 attached to the muscles, which will respond to a feeble electric stimulus. 

 It is by experiments made on frogs prepared in this way, that most of 

 the important facts in regard to the action of electricity on the nervous 

 system have been developed. 



In physiological experiments it is sometimes necessary to use differ- 

 ent forms of electrical apparatus in order to study different properties 

 and phenomena of nerve and muscle. A description of the apparatus 

 thus used would be out of place in this work, and it will be necessary 

 only to enumerate and describe the different currents used and the 

 manner of their application. Many of the phenomena, also, described 

 by electro-physiologists, although curious and interesting, have little 

 apparent application to human physiology or to the practice of medi- 

 cine. A description of such phenomena may well be brief in a work 

 for the use of students and practitioners of medicine. 



In studying the action of nerve and muscle, experimenters often use 

 what is called a single faradic, or induction shock, with a duration of 

 about yoVo (0.0008) of a second. The excitation, therefore, is practically 

 instantaneous. These single shocks are produced by Du Bois-Reymond's 

 apparatus., which is a modification of the faradic, or induction battery. 

 It will be seen farther on that somewhat different effects are produced 

 by the stimulus, due to closing and opening the circuit, and that with a 

 feeble current, no contractions occur at any other time. The contrac- 

 tions thus produced are known respectively as opening and closing. By 

 the use of Du Bois-Reymond's keys, either the closing or the opening 

 excitation may be diverted from the nerve, and a single closing or open- 

 ing shock may be applied at will. 



