CHEMICAL SIGNS OF IRRITABILITY 45 
and also reduces the time during which a current of any 
strength can stimulate the nerve. Exhaustion comes 
on much more rapidly in a hydrogen atmosphere. 
Frohlich found that when a sciatic nerve of a frog is 
deprived of atmospheric oxygen its irritability, measured 
by the threshold of stimulation for muscular contraction, 
decreases more and more, until after the lapse of some 
hours the stimulation required is so strong as to approach 
the region where electrical currents spreading down the 
nerve stimulate the muscle directly. If such is the 
case in a frog’s nerve, the claw nerve, too, left in hydrogen 
may in reality not be stimulated by such a weak current. 
Thorner, also, taking the action current as an index, 
found that a nerve continuously stimulated in an atmos- 
phere deficient in oxygen was quickly exhausted. It 
is remarkable that the action current of a nerve in 
nitrogen gas falls to two-thirds of its original value 
within the first ten minutes. Fatigue of the nerve by 
continuous stimulation during the first few minutes of 
our experiments with hydrogen may then have been 
brought about. 
Whatever interpretation we take—and, as a matter 
of fact, both factors doubtless enter here—the fact that 
there is no decided increase of carbon dioxide on weak 
electrical stimulation in hydrogen points inevitably to 
the view that oxygen is a primary factor in the excita- 
bility of the nerve, as well as in the conduction of the 
nerve impulse. 
Recently Bayliss has pointed. out what he considers 
a probable error in our experiments. To him it seems 
that the increased production of carbon dioxide on 
electrical stimulation may be due, in consequence of the 
