STIMULATION OF MUSCLE AND NERVE 127 



STIMULATION OF HUMAN NERVES 



Duchenne devised a method by which either the 

 motor or the sensory human nerves can be stimu- 

 lated at will, and the reaction of single muscles 

 or groups of muscles to electricity determined. 

 When electrodes are placed on the surface of the 

 skin and the circuit is made, the current entering 

 at the anode will spread in current lines through 

 the entire body. At the cathode, all these lines 

 will converge again. The density of the current 

 depends on the concentration of the current 

 lines. Thus the density is relatively great at 

 the electrodes, and becomes rapidly weaker as 

 the lines diverge between them. The smaller the 

 electrode, the greater the density. The stimulat- 

 ing effect depends on the density. With small 

 electrodes, a current not sufficient to cause stimu- 

 lation may gradually be increased in strength 

 until the density at the electrode becomes great 

 enough to stimulate, while in all other regions it 

 is not yet great enough. Thus a local stimula- 

 tion is secured. But this local stimulus does 

 not sufficiently distinguish between the sensory 

 nerves and the motor nerves and muscles ; for in 

 order to reach the deeper lying motor nerves and 

 muscles, the current must pass through the skin. 

 The resistance of the epidermis is very great, and 



