CHEMICAL SIGNS OF IRRITABILITY 13 



a part which is in activity and the other on a part which 

 is less active, it \vill be found that there is a current 

 which flows in the tissue from the more active to the less 

 active part, from the more to the less excited, and outside 

 in the galvanometer from the less to the more excited 

 part. This electrical current was discovered by Galvani 

 and is called the current of action, or the action current. 

 Generally, when an impulse sweeps along a nerve to 

 which two electrodes are applied, first one electrode 

 and then the other becomes negative, so that the current 

 is diphasic, running first in one direction and then in the 

 other, Now, Waller observed that when a nerve was 

 exposed to carbon dioxide this diphasic current showed 

 a characteristic change, the negative phase being first 

 increased by small amounts of carbon dioxide and then 

 diminished. He then discovered that just the same kind 

 of a change occurred in the electrical response if he stimu- 

 lated a nerve repeatedly at very short intervals of time. 

 He concluded from this that on stimulation of the nerve 

 carbon dioxide was produced, and that this caused the 

 characteristic alteration of the electrical response which 

 occurred in the tetanized or repeatedly stimulated nerve. 

 This conclusion was not generally accepted by physiol- 

 ogists for the reason that it was possible that the same 

 change in the electrical response might be produced in 

 other ways than by carbon dioxide, and while the experi- 

 ments were regarded as circumstantial evidence of value, 

 showing that a chemical change accompanied the nerve 

 impulse, they were not regarded as conclusive. 



Waller supported this conclusion by another dis- 

 covery, namely, that when he stimulated the nerve at 

 regular intervals, not too long or too short, by a strong 



