ACTION OF ELECTRICITY UPON THE NERVES. 533 



the muscle is thrown into a tetanic condition. This is called " closing teta- 

 nus." When a constant current, not of sufficient strength to produce closing 

 tetanus, is passed for several minutes through a long extent of nerve, a very 

 vigorous contraction occurs on opening the circuit, which is followed by teta- 

 nus lasting for several seconds. This is called " opening tetanus." After a 

 time, this varying with the excitability of the nerve and the strength of the 

 current, the descending current will destroy the nervous excitability, but it 

 may be restored by repose, or more quickly by the passage of an ascending 

 current. If the ascending current be passed first for a few seconds, a con- 

 traction follows the opening of the circuit ; and this contraction, within cer- 

 tain limits, is more vigorous the longer the current is passed. At the same 

 time, the prolonged passage of the ascending current increases the excitabil- 

 ity of the nerve for any kind of stimulus. 



After a certain time, which varies in different animals, the nervous excita- 

 bility becomes somewhat enfeebled by exposure of the parts. The phenom- 

 ena then observed belong to the conditions involved in the process of " dying " 

 of the nerve. In the later stages of this condition, the phenomena may be 

 formulated as follows : 



If the sciatic nerve attached to the leg of a frog, prepared in the usual 

 way for such experiments, be subjected to a feeble galvanic current, there is 

 a time when muscular contraction takes place only at the instant when the 

 circuit is closed, no contraction occurring when the circuit is opened ; and 

 this occurs only with the descending current. With the ascending current, 

 contraction of the muscles occurs only when the circuit is opened and none 

 takes place when the circuit is closed. These phenomena are distinct after 

 the excitability of the parts has become somewhat diminished by exposure or 

 by electric stimulation of the nerve. 



If a sufficiently powerful constant current be passed through a nerve, dis- 

 organization of its tissue takes place, and the nerve finally loses its excita- 

 bility, as it does when bruised, ligatured, or when its structure is destroyed 

 in any other way. It was thought by Galvani, and the idea has been adopted 

 by Matteucci, Guerard and Longet, that a current directed exactly across a 

 nerve, so as to pass at right angles to its fibres, does not give rise to muscular 

 contraction. This view is generally accepted by physiologists. 



The muscular contraction produced by electric stimulation of a nerve is 

 more vigorous the greater the extent of the nerve included between the poles 

 of the battery. This fact has long been observed, and its accuracy may easily 

 be verified. It would naturally be expected that the greater the amount of 

 stimulation, the more marked would be the muscular action ; and the stimu- 

 lation seems to be increased in proportion to the extent of nerve through 

 which the current is made to pass. 



The excitability of a nerve, it is well known, may be exhausted by the 

 repeated application of electricity, whatever be the direction of the current, 

 and it is more or less completely restored by repose. When it has been ex- 

 hausted for the descending current, it will respond to the ascending current, 

 and vice versa ; and alter it has been exhausted by the descending current, 



