STIMULATION OF MUSCLE AND NERVE 111 



they are so joined with each other as to make it 

 impossible to pass a current through the heart 

 muscle without the current lines cutting fibres 

 in every direction. It would seem therefore that 

 secondary anodes and cathodes would be formed 

 to such a degree that the demonstration of polar 

 excitation would be difficult or impossible. Ex- 

 perimentation shows however that this is not the 

 case. The heart behaves like a single hollow 

 fibre. 



Monopolar Method. The small size and conical 

 form of the ventricle of the frog's heart make the 

 ordinary method of stimulation, in which the 

 electrodes would both be placed on the heart, 

 less suitable than the monopolar method. This 

 method was suggested by the fact that the 

 stimulating effect of the galvanic current depends 

 on its density. If one electrode has a large sur- 

 face, and the other a very small surface, the 

 current lines will be distributed through a con- 

 siderable cross-section in the first instance and 

 converge to a small cone in the second. The 

 threshold value of stimulation will not be 

 reached at the large electrode, and stimulation 

 will occur only at the small electrode. Thus the 

 large "indifferent" electrode may be placed on 

 any part of the frog's body, and the convenient 

 small electrode be used to stimulate the heart. 



