CHEMICAL SIGNS OF IRRITABILITY 13 
a part which is in activity and the other on a part which 
is less active, it will 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 
