XII. Stimulation with the constant current. The 

 simple rheocord. 



/. Appliances. Operating case, kymograph and myograph; 

 3 or 4 Daniell cells; simple rheocord; materials for mak 

 ing nonpolarizable electrodes, (see demonstration 

 VIII); Pohl's commutator with cross-bars; Du Bois 

 Reymond key; 9 wires; 3 frogs. 



2. Preparation. 



(1.) Make a pair of N P electrodes. 



(2.) Set up apparatus as shown in PI. II, Fig. 6. 



j. Operation. Make and mount a gastrocnemius prepc'. 

 ration and so adjust the nerve to the electrodes that the: 

 current will be a "descending" one, i. e. so that the 

 kathode will be nearer to the muscle than is the anode 



4.. Observations. 



(1) (a) Open the short-circuiting Du Bois-Reymond 

 key i. e. make the long circuit. 



(b) Close the key, thus breaking the long circuit, or 

 muscle-circuit. 



(c) Take a tracing of a series of alternating make and 

 break shocks with descending current. 



(d) Take a tracing with ascending current. How may 

 one change the direction of the current along the 

 nerve without changing the adjustment of nerve 

 and electrodes? 



(2) (a) Give the preparation a stronger stimulus by 

 joining two cells. Should one join the cells in series 

 or multiple arc ? Why ? 



(b) Take a tracing as before using the descending 

 current. 



