STRUCTURE AND PHYSICAL PROPERTIES OF MUSCLE 535 



point where the current enters the muscle. Tne alteration of concen- 

 tration of electrolytes under the kathode heightens, while that under 

 the anode depresses, excitability during the passage of the current. 

 When the current is broken, the effects are reversed (see p. 533). 



To alter the strength of a constant current, a piece of apparatus 

 known as the rheocord is used. In its simplest form this consists 

 of a wire wound to and fro across a board, with a terminal at either 



FIG. 268. To ILLUSTRATE: THE PRINCIPLE OF THE RHEOCORD. 



end and a movable contact or slider between them (Fig. 208). In 

 use, the cell is connected to the two terminals A, B, and the prepara- 

 tion connected to the end A, at which the current enters, and to the 

 slider S. The current from the cell can now pass either through 

 the preparation or back along the wire of the rheocord. The amount 

 which will pass in either direction is determined by the position of S. 

 The fall of potential in the rheocord is from .4 to B; therefore, when 



s.c -, 



FIG. 269. DIAGRAM OF AN INDUCTION COIL AND ITS CONNECTIONS. 



S is near to A, the fall of potential from A to S is small, and but little 

 of the current will pass to the preparation. The amount of current, 

 therefore, going to the preparation is directly proportional to the fall 

 of potential between .4 and S. It is also inversely proportional to 

 the resistance of the circuit through the nerve. This resistance, 

 however, need not be considered, since the resistance in the nerve 

 is so many times greater than that caused by any change in the position 

 *& 



