g8 A CHEMICAL SIGN OF LIFE 



found to run in nerves between the two cut ends of the 

 nerve, up or down it, as the case may be. Evidently 

 electrical current is generated in a nerve between two 

 parts which are unequally irritable, or unequally under- 

 going chemical change. The part of the nerve which 

 is respiring most is in a different electrical state from that 

 which is respiring less, and thus we see the very clear 

 and definite relationship between the chemical and the 

 electrical changes which have been particularly dwelt 

 upon by Waller. This is one of the most important and 

 fundamental discoveries which we have noted, for it 

 means that there must be a decrement in the rate of the 

 impulse as it flows down the fiber, and that the distance 

 to which a nerve impulse can be transmitted is not 

 indefinite, but that that impulse diminishes as it pro- 

 ceeds, and will ultimately die out. In the medullated 

 nerves, to be sure, this decrement is not large, for it is 

 very necessary that it should be as small as possible in 

 the more highly developed nerves; but it is to be found 

 everywhere. And in simple undifferentiated proto- 

 plasm of plants and animals it is easily shown to exist. 

 The more rapidly nerves respire the faster do they 

 appear to carry the impulse; irritability and the rate 

 of production of carbon dioxide in resting nerves thus 

 appear to be correlated. The more respiration the 

 more life! If we abolish respiration temporarily, or 

 reduce it, we find that irritability has been reduced in 

 somewhat the same proportion. Anesthetized nerves 

 of all kinds show a reduced output of carbon dioxide, 

 and they recover their irritability when they breathe 

 again. Anesthetics do not, therefore, affect the physical 

 state of the protoplasm only, as they have been supposed 



