THE PHENOMENON OF CONDUCTION. 91 



other hand, when very strong currents are used the stimuh that act 

 at the two poles set up nerve impulses w^hose passage to the muscle 

 may be blocked by the depressed conductivity caused by the electro- 

 tonic changes. Whether or not the stimulus will be effective in 

 causing a contraction in the attached muscle will depend naturally 

 on the relative positions of the electrodes, — that is, on the direction 

 of the current in the nerve. In describing the effect of these strong 

 currents we must distinguish between what are called ascending 

 and descending currents. Ascending currents are those in which 

 the direction of the current in the nerve is away from the muscle, 

 a position of the poles, therefore, in which the anode is closer to 

 the muscle. In descending currents the positions are reversed. 

 Pfliiger's law of contraction or of stimulation takes account of 

 the effect of extreme variations in the strength of the current 

 and is usually expressed in tabular form as follows: The letter C 

 indicates that the nerve is stimulated and causes a contraction in 

 the attached muscle, and indicates a failure in the stimulation 

 (weak currents) or a failure in the nerve impulse to reach the muscle 

 owing to blocking (strong currents) . 



Fig. 33. Fig. 34. 



Figs. 33 and 34. — Schema to show the arrangement of apparatus for an ascending and a 

 descending current: Fig. 33, ascending; Fig. 34, descending. 



Ascending Current. Descending Current. 



Making. Breaking. Making. Breaking. 



Very weak currents . . C O C O 



Moderate " C C C C 



Very strong " O C C O 



The effects obtained with the strong currents are readily under- 

 stood if we bear in mind the facts stated above regarding electro- 

 tonus. When the current is ascending the stimulus on making 

 starts from the cathode, but cannot reach the muscle because it 

 is blocked by a region of anelectrotonus in which the conduc- 

 tivity is depressed. The stimulus on breaking takes place at 



