LAW OF CONTRACTION 487 



impulse, occurring at the anode, which is nearer the muscle, produces 

 an opening contraction, and the closing impulse, which is at the cathode, 

 is not conducted to the muscle. 



While the constant current usually does not excite contractions dur- 

 ing the actual time of its passage through a nerve, with a certain strength 

 of current the muscle is thrown into a tetanic condition. This is called 

 "closing tetanus." When a constant current, not of sufficient strength 

 to produce closing tetanus, is passed for several minutes through a long 

 extent of nerve, a vigorous contraction occurs on opening the circuit, 

 which is followed by tetanus 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 ascend- 

 ing current is passed first for a few seconds, a contraction follows the 

 opening of the circuit ; and this contraction, within certain limits, is 

 more vigorous the longer the current is passed. At the same time, the 

 prolonged passage of the ascending current increases the excitability of 

 the nerve for any kind of stimulus. 



After a certain time which varies in different animals the ner- 

 vous excitability becomes somewhat enfeebled by exposure of the parts. 

 The phenomena 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, is subjected to a feeble galvanic cur- 

 rent, 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 is passed through a nerve, 

 disorganization of its tissue takes place, and the nerve finally loses its 

 excitability, as it does when bruised, ligatured, or when its structure is 

 destroyed in any other way. It was thought by Galvani 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 commonly 

 accepted by physiologists. 



The muscular contraction produced by electric stimulation of a nerve 



