POLAR STIMULATION 



35 



2 volts the potentiometer is better omitted, since the 

 branch current is not strong enough to give marked 

 local contraction.) The electrodes consist of two wires 

 passed, about 2*5 cm. apart, through a piece of cork. 



Pin the rectus abdoniinis, at about its normal length, 

 across the electrodes. Put the slider at 20 of the rheo- 

 chord scale. Turn the moveable bar of the commutator 

 so that the positive pole of the battery is connected 

 with the right-hand electrode. Watch one of the middle 



Fig. 16. Diagram of rectus abdominis muscle of the frog, showing in 

 the middle portion the position in which prolonged contraction 

 occurs according to the direction of the galvanic current. 



tendinous insertions of the muscle, best with a lens and 

 by oblique light, and sharply close the in-circuit key 

 and in two seconds open it. Move the slider to the left 

 10 degrees at a time, and stimulate as before after each 

 move. The first effect will be a twitch at make and break ; 

 at some stage of increase in the strength of the current, 

 a wheal will be formed on the right-hand side of each 

 tendinous insertion, i.e. a protracted contraction is pro- 

 duced at the cathode of each segment (cf. diagram, Fig. 

 16, A). This contraction differs from the twitch in being 



3 2 



