THE ELECTRICAL CONDITIONS OF MUSCLE AND NERVE 6l 



Secondary contraction from the heart. Lay the nerve of a muscle- 

 nerve preparation upon the beating heart of the frog. If the 

 preparation is very excitable the muscle will twitch with each beat 

 of the ventricle. If the heart-beat and the twitch are simultaneously 

 recorded on a drum the twitch will be found to slightly precede the 

 beat i.e., the electrical change precedes the mechanical ; this is seen 

 best with a cooled heart. 



Measurement of demarcation current ; Capillary electrometer. 

 The capillary electrometer consists of a thread of mercury, which is 

 forced by pressure from behind for a certain distance along a glass 

 tube drawn out to a capillary termination ; the free end of the capillary 

 is filled with dilute sulphuric acid and dips into a vessel containing 

 the same fluid. The capillary is observed with a microscope. If the 



FIG. 52. Experiment to show secondary contraction, k, Mercury key in primary 

 circuit; k l , short-circuit key in secondary circuit; a, first muscle ; 6, second 

 muscle with its nerve laid over the first. 



mercury and the sulphuric acid be now connected with wires which 

 are charged with electricity, there is produced a movement of the 

 mercury in the direction which the current would take i.e., from 

 positive to negative the extent of movement through a small range 

 being, for the same electrometer, proportional to the difference of 

 potential. From the direction and extent of the movement the 

 direction and electromotive force of any constant current can therefore 

 be gauged. The movements of the meniscus can also be photographed, 

 and a graphic record thus obtained. 



Join a pair of non-polarisable electrodes up in circuit with a 

 capillary electrometer and Daniell cell through a rheochord and com- 

 mutator in the manner shown in the diagram (Fig. 53), but with 

 a piece of blotting-paper moistened with salt solution placed across the 

 electrodes instead of the muscle shown in the figure. Have the short- 

 circuiting key (k l ) shut at first so that the electrometer is short-circuited, 



