THE EXCITATION OP NERVE FIBRES 269 



tion of the current cause slight apparent irregularities in the law of contrac- 

 tion as tested on man. 



In stimulating the nerves of man for the purpose of determining the con- 

 ditions of the different muscles, we may use either induced currents (generally 

 called faradic stimulation) or the make and break of a battery current (galvanic 

 stimulation). The electrodes are covered with 

 chamois leather moistened with salt solution 

 in order to diminish the resistance of the skin. 

 When it is desired to stimulate any given muscle 

 the stimulating electrode is brought as nearly as 

 possible over the spot where the muscle receives 

 its motor nerve. These ' motor points ' have been 

 mapped out, and reference is generally made to a 

 diagram in determining the point for any given 

 muscle. By reversing the current the stimulating 

 electrode may be made either anode or cathode. 

 It is found that stimulation occurs most easily 

 on closure of the current when the stimulating 

 electrode is the cathode ; with the greatest diffi- 

 culty when the current is broken and the stimu- 

 lating electrode is the cathode. These different 

 contractions are generally represented by capital 

 letters, and the usual relationship is ex- 

 pressed by the statement that CCC is obtained F IG . 118. Electrodes applied to 



most easily, then ACC and AOC, and finally 

 COG. 



CCC = cathodal closing contraction. 



ACC = anodal closing contraction. 



AOC = anodal opening contraction. 



COG = cathodal opening contraction. 



the skin over a nerve-trunk. In 

 A the polar area is anelectrotonic 

 and the - peripolar catelectro- 

 tonic. The former condition 

 therefore preponderates, since 

 the current here is more con- 

 centrated. In B the conditions 

 are reversed, the polar zone 

 corresponding in this case to the 

 cathode. (WALLER.) 



When the motor nerve to a muscle has under- 

 gone degeneration the muscle also begins to 

 degenerate, and we find certain alterations in its response to . artificial i 

 lation. In the first place, the muscle may fail to respond to induction shocks, while 

 may show an increased irritability for galvanic shocks. In the second place, qualita- 

 tive alterations in irritability may be present, so that ACC may be obtained witJ 

 smaller current than CCC. These alterations are spoken of as the ' reacti 

 tion.' 



