CURRENTS IN MUSCLE .AND NERVE. 23 7 



galvanometer circuit. Note, from the direction in, which the 

 spot moves, which of the two points of the muscle touched 

 by the electrodes is positive to the other. 



Now stimulate the nerve with interrupter shocks of just 

 sufficient strength to tetanise the muscle, and observe that 

 the previous deflection is diminished. The action current is, 

 opposed in direction to the demarcation current. 



Next cut off the end of the muscle near the tendon, and 

 apply the electrode to the artificial cross section ; the deflection 

 may be so great as to throw the spot off the scale. Bring 

 it back by putting a plug into the shunt so that only 

 a fraction, i.e., 1/10, 1/100, or 1/1,000, passes to the galvano- 

 meter. The shunt makes a proportionate short circuit. 



Again stimulate the nerve, the movement of the spot in 

 the contrary direction will be more marked. 



Currents in the frog's heart. Excise the entire heart, in- 

 cluding the sinus, dispose the electrodes close to each other 

 so that the heart may rest with its base upon one of them, 

 and the apex on the other. 



Close the galvanometer circuit between the beats and observe 

 that there is a deflection (demarcation current), which under- 

 goes a sudden diminution at each contraction (action current). 



Current from the nerve. Remove the two sciatic nerves,, 

 cut them as long as possible, and arrange them across one 

 electrode with both ends of each hanging down and touching 

 the second electrode. 



Close the galvanometer circuit and note the deflection. 

 The longitudinal surfaces of the nerves are positive to 

 their ends. 



The cut end of a nerve is in a state of katelectrotonus, 

 due to changes accompanying the death of the nerve. 



Capillary electrometer (Fig. 47, 2). Repeat the previous experi- 

 ment with this instrument instead of the galvanometer. 



