THE ELECTRICAL CONDITIONS OF MUSCLE AND NERVE 59 



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 aifect the electrometer. From the direction of move- 

 ment 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 rheocord) as to bring the mercury back 

 to zero. 



CJCC 



FIG. 52. DIAGRAM OP CAPILLARY ELECTROMETER. A, RESERVOIR CONTAINING SULPHURIC ACID. 

 s, AND MERCURY, m' ; m, MERCURY IN GLASS TUBE DRAWN OUT TO CAPILLARY TERMINA- 

 TION ; B, CAPILLARY AS SEEN UNDER MICROSCOPE ; n.p. NON-POLARISABLE ELECTRODES ; exc, 

 EXCITING ELECTRODES ; k, A-', KEYS ; c, COMMUTATOR ; rl>, RHEOCHORD. 



Action-current. Place the upper end of the nerve of the preparation on a 

 pair of exciting electrodes connected with an induction coil arranged for tetanisa- 

 tion. Observe the meniscus with the microscope, and tetanise the muscle, 

 using the weakest possible stimulus. Notice that the meniscus moves in a 

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

 condition of the muscle accompanying its contraction. From the direction 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 plate. 



String galvanometer of Einthoven. This consists of a microscopically fine 

 thread of silvered quartz stretched between the poles of a powerful electro- 

 magnet (fig. 53). When a galvanic current is passed along it the thread 



