CHAP. IL] THE CONTRACTILE TISSUES. 61 



current as we shall call it, and the induced current obtained from 

 the constant current by means of an induction coil, as it is called ; 

 for the physiological effects of the two kinds of current are in 

 many ways different. 



It may perhaps be worth while to remind the reader of the following- 

 facts. 



In a galvanic battery, the substance (plate of zinc for instance) 

 which is acted upon and used up by the liquid is called the positive 

 element, and the substance which is not so acted upon and used up 

 (plate &c. of copper, platinum, or carbon, &c.) is called the negative 

 element. A galvanic action is set up when the positive (zinc) and the 

 negative (copper) elements are connected outside the battery by some 

 conducting material, such as a wire, and the current is said to flow in a 

 circuit or circle from the zinc or positive element to the copper or 

 negative element inside the battery, and then from the copper or negative 

 element back to the zinc or positive element through the wire outside 

 the battery. If the conducting wire be cut through, the current ceases 

 to flow ; but if the cut ends be brought into contact, the current is re- 

 established and continues to flow so long as the contact is good. The 

 ends of the wires are called 'poles,' or when used for physiological 

 purposes, in which case they may be fashioned in various ways, are 

 spoken of as electrodes. When the poles are brought into contact or 

 are connected by some conducting material, galvanic action is set up, 

 and the current flows through the battery and wires ; this is spoken of 

 as "making the current" or " completing or closing the circuit." When 

 the poles are drawn apart from each other, or when some non-conducting 

 material is interposed between them, the galvanic action is arrested ; 

 this is spoken of as " breaking the current" or " opening the circuit." 

 The current passes from the wire connected with the negative (copper) 

 element in the battery to the wire connected with the positive (zinc) 

 element in the battery; hence the pole connected with the copper 

 (negative) element is called the positive pole, and that connected with 

 the zinc (positive) element is called the negative pole. When used for 

 physiological purposes the positive pole becomes the positive electrode, 

 and the negative pole the negative electrode. The positive electrode is 

 often spoken of as the anode (ana, up), and the negative electrode as 

 the kathode (kata, down). 



A piece of nerve of ordinary length, though not a good conductor, 

 is still a conductor, and when placed on the electrodes, completes the 

 circuit, permitting the current to pass through it ; in order to remove 

 the nerve from the influence of the current it must be lifted off from 

 the electrodes. This is obviously inconvenient ; and hence it is usual 

 to arrange a means of opening or closing the circuit at some point along 

 one of the two wires. This may be done in various ways : by fastening 

 one part of the wire into a cup of mercury and so by dipping the other 

 part of the wire into the cup to close the circuit and make the current, 

 and by lifting it out of the mercury to open the circuit and break the 

 current ; or by arranging, between the two parts of the wires, a 

 moveable bridge of good conducting material such as brass, which can 

 be put down to close the circuit or raised up to open the circuit ; or in 



