THE ELECTRICAL CONDITIONS OF MUSCLE AND NERVE 63 



may be connected. Open the battery key and close the short-circuiting 

 key ; the meniscus should return to its original position. 



Lay the muscle of a nerve-muscle preparation, which may have the 

 distal end cut or injured, upon the electrodes in place of the wet 

 blotting-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 i.e., which part of the muscle led off from the electrodes is 

 negative to the other. 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 rheochord and commutator 

 sending a current through the circuit 

 in a direction the reverse of the 

 demarcation current and of exactly 

 such a strength (measured by the 

 known electromotive force of the 

 battery employed and the position 

 of the rider on the rheochord) as to 

 bring the mercury back to zero. 



Action current. Place the upper 

 end of the nerve of the preparation 

 on a pair of exciting electrodes (exc) 

 connected with an induction coil 

 arranged for tetanisation. Observe 

 the meniscus with the microscope, 

 and tetanise the muscle, using the 

 weakest possible stimulus. Notice 

 that the meniscus moves in a par- 

 ticular direction. This movement is caused by a change in the electrical 

 condition of the muscle accompanying its contraction. From the direc- 

 tion of movement of the mercury determine which part of the muscle 

 is now negative to the other. The action current can only be properly 

 studied in photographs of the end of the mercury column, the image of 

 which is thrown on a slit in front of a moving photographic plate. 



String galvanometer of Einthoven. This consists of a microscopically 

 fine thread of silvered quartz or of wire stretched between the poles of 

 a powerful electro-magnet (Fig. 54). When a galvanic current is passed 

 along it the thread is deflected to one side or the other to an extent 

 varying with the E.M.F. the tension of the thread being supposed 

 constant. The movement is observed with a microscope, or the mag- 



FIG. 54. Diagram of Einthoven 

 string galvanometer, s, s', Wire or 

 silvered quartz thread, stretched 

 between the two poles of a power- 

 ful electro-magnet, TO, m, which 

 are perforated to allow a micro- 

 scope to bear upon the thread. 



