PRACTICAL PHYSIOLOGY 57 



Open the battery key and close the short-circuiting 

 key ; the meniscus should return to its original posi- 

 tion. Lay a muscle, which may have one end cut or 

 injured, upon the electrodes in place of the wet blot- 

 ting paper. Place it with one electrode touching 

 the longitudinal surface and the other at or near 

 the injured end. Then open the short-circuiting 

 key to allow the demarcation current of the muscle 

 to affect the electrometer. From the direction of 

 movement of the mercury determine the direction 

 of the muscle current through the apparatus. The 

 electromotive force of the current can be measured 

 by closing the battery key, so that the battery 

 current is brought into the circuit, and by aid of 

 the rheocord and commutator sending a current 

 through the circuit in a direction the reverse of the 

 demarcation current and of exactly such a strength 

 (as measured by the position of the rider on the 

 rheocord) as to bring the mercury back to zero. 



G-alvaiiometer. Substitute in the above experi- 

 ment a high-resistance galvanometer (Fig. 39) for 

 the electrometer, and repeat the above observa- 

 tions, using the movement of the needle as the 

 index instead of the mercury of the electrometer. 



Having with either the electrometer or galvano- 

 meter determined the existence and direction of a 

 current in muscle, tetanize a muscle through its nerve 

 as it lies upon the non-polarisable electrodes, and no- 

 tice the diminution of the excursion of the mercury 

 or of the magnetic needle which occurs on stimula- 

 tion (negative variation of the demarcation current). 



.The demarcation and excitation currents of nerve 



