220 HUMAN PHYSIOLOGY 



The electric current does not stimulate if it passes trans- 

 versely through the nerve. 



In Fig. 1 8 let TV be the nerve of a muscle-nerve preparation, and 

 let the current enter at -f- and leave at . Determine the latent 

 period for the make and break contraction 

 (in a similar manner as in determining the 

 velocity of the impulse, page 219). It will 

 be found that the latent period of the make is 

 greater than that of the break contraction. 

 If now the current is passed through the nerve 

 FlG - l8 - in the opposite direction, it will be found that 



the latent period of the make is smaller than that of the break 

 contraction. The difference in these, latent periods corresponds 

 to the time taken by the impulse to travel through the piece of 

 nerve between the two electrodes. This proves that the stimula- 

 tion of the making of the current takes place at the kathode, while 

 that of the breaking occurs at the anode. 



The changes in the irritability which the current produces at 

 the electrode are investigated as follows: 



The nerve is stimulated near either electrode of the constant 

 current by a stimulus of constant strength, first before the con- 

 stant current is passed through the nerve, and then while the 

 constant current is passing through the nerve. Observe whether 

 the contraction in the second case is larger or smaller than that in 

 the first. 



The irritability is changed during the entire passage of the 

 current. This condition of changed irritability produced by 

 the current is called electrotonus ; the condition of increased 

 irritability at the kathode is called katelectrotonus, the con- 

 dition of decreased irritability at the anode is called anelec- 

 trotonus. We may express these observations in the law: 

 The appearance of katelectrotonus and the disappearance of 

 an electrotonus stimulate. 



The decreased conductivity occurring at the anode during 

 the making and, for a short period, at the kathode during 

 breaking, is so great, when strong currents are used, that the 

 nerve at these places loses its conductivity altogether. If a 

 part of a nerve which has lost its conductivity is situated 

 between the place stimulated and the end-organ (muscle), 

 the stimulation has no result. -This occurs in the following 

 cases: 



