178 MUSCLE AND NERVE 



stimulated. Any change in the strength of the primary current 

 i. e., the current furnished by the voltaic cell brings about 

 a change of potential in the outer coil, which manifests itself 

 as the induced or secondary current. This gives a strong shock 

 of momentary duration and produces less injury than the 

 voltaic current. 



The shock resulting from closure of the primary current is 

 called the closing shock, while that from the opening is called 

 the opening shock. With the constant current the former 

 is more effective than the latter. The opposite is true of the 

 induced current. The cause of this difference lies in the con- 

 struction of the induction coil. As the current passes along 

 each turn of wire forming the primary coil it excites an "in- 

 duced" current in the neighboring turn of wire, which, having 

 an opposite direction to the primary current, opposes it, and 

 therefore delays it in reaching its maximum strength; but, 

 when the primary current is broken, both it and the induced 

 currents in the neighboring turns of wire have the same direction 

 and do not oppose each other. Since the intensity of the induced 

 current with the same strength of primary current depends 

 upon the rapidity with which the primary reaches its maximum, 

 it follows that the closing induced current must be weaker 

 than the opening, and have, consequently, a smaller stimu- 

 lating effect. 



The effect of a stimulus is proportional to the rate with 

 which it reaches its maximum. This is true only within limits, 

 for a stimulus may be applied both too slowly and too quickly 

 to produce any effect. This has been expressed in Du Bois- 

 Reymond's law : " It is not the absolute value of the current 

 at each instant to which the motor nerve replies by a contrac- 

 tion of its muscle, but the alteration of this value from one 

 moment to another; and, indeed, the excitation to movement 

 which results from this change is greater the more rapidly it 

 occurs by equal amounts, or the greater it is in a given time." 

 This law is not strictly true. 



The denser the current the greater is its stimulating effect. 

 This is well illustrated by the phenomena of unipolar action. 

 Usually, in order to stimulate a preparation with an electrical 

 current, it is necessary that there shall be a complete circuit; 

 but under certain circumstances one wire of a secondary coil 



