CHEMICAL SIGNS OF IRRITABILITY 45 



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 



