484 NERVOUS SYSTEM 



What is commonly known as an interrupted current is a faradic 

 or induced current, in which the closing and opening excitations follow 

 each other with greater or less rapidity, and the intervals may be regu- 

 lated so that they occur at a regular rate. A rapid succession of induc- 

 tion-shocks produces a more or less prolonged muscular action, called 

 tetanic contraction. The number of successive shocks in a second 

 required to produce a tetanic condition of a muscle varies in different 

 animals and in different muscles in the same animal. The minimum 

 seems to be about sixteen per second, with a considerable range of vari- 

 ation. Very rapid stimuli, even more than 3000 per second, will pro- 

 duce tetanic contraction. 



The faradic, or induced current is different in its effects, under 

 certain conditions of the nerves and muscles, from an interrupted gal- 

 vanic, or primary current. This question is important in practical medi- 

 cine in determining the so-called " reaction of degeneration" of nerve 

 and muscle. 



The constant current, under certain conditions, has no effect that 

 is indicated by muscular phenomena, contraction occurring only on clos- 

 ing or opening the circuit. This is known as the galvanic, or primary 

 current. It produces, however, a peculiar condition of nerves and 

 muscles, which will be described under the head of electrotonus. The 

 primary current is derived directly from the cells of a galvanic battery, 

 and this is to be distinguished from the faradic, or induced current. 

 The faradic current is induced in a coil of small insulated wire, brought 

 near and parallel to and partly or entirely surrounding a coil of larger 

 wire carrying the primary current. When the circuit of the primary 

 current is closed, the direction of the induced current is the reverse of 

 that of the primary current. When the primary circuit is opened, the 

 induced current has the same direction as the primary current. The 

 direction of the primary current is uniform, but the direction of 

 the induced current alternates with each interruption of the primary 

 current. These induced currents are of momentary duration, being 

 produced only when the primary current is closed and opened. A rapid 

 interruption of the primary current is produced by what is called a 

 rheotome, or current-interrupter, which is attached to all induction- 

 batteries. 



The points or surfaces used in closing a circuit within which a 

 portion of nerve or muscle is included are called electrodes. They 

 usually are designated as the copper, or positive electrode or pole, and 

 the zinc, or negative electrode or pole. The positive pole is also called 

 the anode, and the negative pole, the cathode. The direction of the 

 current, when the circuit is closed, is from the anode to the cathode. 



