MUSCLE CURRENTS 151 



current of known direction through the galvanometer. For this 

 purpose arrange the shunt to send r oV^th of the current through the 

 galvanometer and take a small battery made of a zinc and copper 

 wire with wires soldered to them, passing through a cork, and dipping 

 into some weak sulphuric acid in a small tube. The short-circuiting 

 key of the shunt is closed, and then the leads are connected to the 

 galvanometer. On opening the short circuit the spot of light is 

 immediately deflected to one side, because the two wires form a little 

 battery of which the copper is the positive pole and the zinc the 

 negative. Note which galvanometer terminal is connected to the 

 copper wire ; you will then know that if in the experiment a deflection 

 occur in the same direction the current causing it enters at that same 

 terminal. Let us suppose that when the spot of light is deflected 

 to the north, the north terminal of the galvanometer is connected 

 with the copper wire, and is therefore positive. 



A pair of unpolarisable electrodes are now prepared and tested to 

 show that they are iso-electric by connecting them to the galvano- 

 meter and bringing the two clay guards in contact, when no deflection 

 must occur. If there be a slight deflection the electrodes may often 

 be rendered iso-electric by connecting their two terminals with a 

 stout copper wire and then placing the two clay guards in contact, 

 leaving them so for some hours. If much deflection be caused it is 

 better to remake the electrodes. 



Now make a semimembranosus and gracilis preparation, taking 

 great care to injure it as little as possible. Place this across the 

 unpolarisable electrodes, so that the centre rests on one electrode and 

 the lower end upon the other. Open the key of the galvanometer 

 and observe if any deflection be produced. It will usually be but slight, 

 and in such direction to show that the tibial end is electro-negative 

 to the equator. Now remove the muscle and injure the tibial end by 

 touching it with a hot wire. Replace the muscle on the electrodes 

 and again open the short-circuiting key of the galvanometer. A 

 deflection is now obtained in such direction as to show that the 

 injured end is negative to the non-injured surface. This is the 

 current of injury. Next apply a pair of electrodes to the upper end 

 of the muscle and tetanise it. The galvanometer swings in the 

 reverse direction and may nearly reach the zero of the scale. This is 

 the current of action, and its direction shows that the injured end 

 has become less electro-negative relatively to the centre of the 

 muscle. 



It is further to be observed that the diminution of current persists 

 throughout the whole time the muscle is tetanically contracted, though, 

 as previously found (p. 148), the current of action is of an alternating 



