226 PHYSIOLOGY 



of the magnet, due to the current of rest or demarcation current. If now the 

 nerve be stimulated with an interrupted current so as to throw the muscle 

 into a tetanus, the ray of light from the galvanometer mirror swings back 

 towards the zero of the scale, showing that the current which was present 

 before is diminished. When the excitation of the nerve is discontinued, the 

 galvanometer indicates once more the original current of rest. This 

 diminution of the current of rest during activity of a muscle is spoken of as 

 the ' negative variation' 



In carrying out this experiment it is usual to compensate the demarcation current 

 by sending in a small fraction of the current from a constant cell. The arrangement 

 of the apparatus is represented in the accompanying diagram. Two non-polarisable 



D 



FIG. 80. 



electrodes np are applied to the surface and cross -section of a muscle m. These are 

 connected with the shunt of the galvanometer, one of the wires, however, being con- 

 nected with a Pohl's reverser P, and this in its turn with the shunts. The two end- 

 terminals of the reverser are connected with a rheochord, through the wire of which 

 ab a constant current is passing from the Daniell cell D. By means of the rider c the 

 fraction of current passing through the reverser can be modified to any extent. The 

 key k being open, the muscle is connected with the shunt and galvanometer, and the 

 direction and extent of the swing noticed. The key k is then closed, and by means 

 of the reverser the current is sent through the galvanometer in the opposite direction to 

 the demarcation current, and the rider c shifted until the two currents exactly balance 

 one another, and the needle of the galvanometer returns to zero of the scale. This 

 adjustment is first made, using only Y^^Q-Q of the total current, and then by means of 

 the shunt, T ^-Q , y 1 ^, and finally the whole current is thrown into the galvanometer. 

 If this precaution be not taken, much too large a current may in the first case be sent 

 through the galvanometer, to the detriment of the instrument. If we know the difference 



of potential between the two ends of the wire, the proportion r will give us the E.M.F. of 



the demarcation current. The galvanometer needle having by compensation been 

 brought to zero, stimulation of the nerve at e by interrupted currents causes the needle 

 to swing at once in the opposite direction to the first variation. This swing is the 

 measure of the negative variation or current of action. 



In order to study the electrical changes accompanying a single muscle twitch, 

 it is necessary to employ some instrument which can react much more rapidly than the 



