ADVANCED EXPERIMENTAL PHYSIOLOGY 119 



over the edge (Fig. 119). The sciatic nerve of a pithed frog x is 

 carefully dissected down to the knee, the thigh is cut across, but the 

 leg and foot are left intact. The nerve is so placed that its cut 

 surface is upon one plug and its longitudinal surface upon the other 

 plug. A watch-glass filled with strong saline solution, which is a 

 good conductor of electricity, is suddenly brought in contact with 

 the ends of the kaolin plugs ; thus the circuit is suddenly made and 

 can be suddenly broken by the removal of the watch-glass. If the 

 preparation be very excitable, a twitch is observed at each make and 

 break of the circuit ; the nerve is stimulated when the circuit of its 

 " current of injury " is completed or broken. 



CHAPTER XIV 



THE ELECTROMOTIVE PROPERTIES OF MUSCLE AND NERVE 

 Continued. THE GALVANOMETER AND THE CAPILLARY 

 ELECTROMETER 



DEMONSTRATIONS. The galvanometer and the capillary elec- 

 trometer are delicate instruments which are easily damaged ; they 

 are employed to investigate the electromotive properties of muscle 

 and nerve. The essential experiments upon that subject have 

 already been performed by means of the so-called " rheoscopic frog." 

 In this course, therefore, the experiments with the galvanometer and 

 the capillary electrometer will be demonstrated to the student and 

 only brief details will here be given. 



The Galvanometer employed in these experiments is Kelvin's 

 reflecting galvanometer. It consists of a suspended system of 

 magnets so arranged as to make the system nearly " astatic " ; the 

 magnets are surrounded by coils of many turns of fine insulated wire. 

 The resistance is high, from 5,000 to 20,000 ohms. The movements 

 of the mirror attached to the magnets are indicated by a spot of 

 light upon the scale. 



The amount of current sent through the galvanometer is regulated 

 by means of a shunt, which is a resistance box whereby roth, T i^th, 

 or yoVoth of the total current can be sent through the galvanometer. 



The electric current from the muscle or nerve is led off by means of 

 unpolarisable electrodes, but before an experiment is performed the 

 electrodes are tested, for in most cases they are not perfectly iso- 

 electric. Any small deflection of the galvanometer due to this cause 

 is compensated by a graduated current from a standard battery sent 

 through the galvanometer in the opposite direction. 



Perfectly uninjured muscle and nerve are iso-electric, but they are 

 generally slightly damaged during the process of dissection and 



1 For these preparations the frogs should have been kept cold for some 

 time before the experiment. 



