108 CHANGE IN FORM IN A MUSCLE WHEN IT CONTRACTS [CH. X. 



attached to these are called poles or electrodes, and the pair of electrodes 

 may be conveniently held in a special form of holder. The electrode 

 attached to the positive element (zinc) is called the negative pole or 



kathode ; that attached to the negative ele- 

 ment (copper) is called the positive pole or 

 anode. If now the two electrodes are con- 

 nected together, an electrical, galvanic or 

 constant current flows from the copper to 

 the zinc outside the battery, and from the 

 zinc to the copper through the fluids of the 

 battery ; if the electrodes are not connected 

 the circle is broken, and no current can 

 flow at all If now a nerve or muscle is 

 laid across the two electrodes the circuit is 

 completed, and it will be noticed at the 

 moment of completion of the circuit the 

 muscle enters into contraction; if the 

 muscle is lifted off the electrodes, another contraction occurs at the 

 moment the circuit is broken. The same thing is done more con- 

 veniently by means of a key: fig. 134 represents two common 

 forms of key. A key is a piece of apparatus by which the current 



Fiu. 133. Diagram of a Dauieil's 

 Battery. 



13. .Mercury Key. 



can be allowed to pass or not 

 through the nerve or muscle laid 

 on the electrodes. "When the key 

 is open the current is broken, as in 

 the next figure (fig. 135); when it is closed the current is allowed 

 to pass. The opening of the key is called break ; the closing of the 

 key is called make. A contraction occurs only at make and break, 

 not while the current is quietly traversing the nerve or muscle. 



FIG. 134. A. Du Bois Raymond's Key. 



