IRRITABILITY AS A SIGN OF LIFE 7 



peculiar results were brought out. In the first place, 

 if there are no other organs attached to the nerve it is 

 impossible to determine by casual observation whether 

 or not the nerve has been stimulated, for there is posi- 

 tively no visible physical sign of the vitality in it. Not 

 even with a microscope can any structural change in the 

 tissue be seen. There is, also, no heat change detected 

 with a method which is sensitive to a millionth of a 

 degree Centigrade. There was, before this work was 

 published, no apparent production of carbon dioxide, 

 or any other chemical change in the tissue. These facts 

 seemed to indicate that the functional activity of nerve 

 fibers was in no way associated with any chemical 

 change. This failure of a nerve to show any chemical 

 or structural changes similar to those of muscles had a 

 decisive influence in the formation of ideas concerning, 

 not only the nature of the nerve impulse, but also the 

 nature of irritability in general. For nerve fibers 

 not only show the highest type of irritability of proto- 

 plasm, but they also possess, as stated before, the power 

 of transmitting the state of excitation in the most perfect 

 manner. And all attempts to explain the nature of 

 irritability in general must necessarily account for the 

 peculiarities of the nerve fiber where we find that prop- 

 erty in its highest development. If irritability, excita- 

 tion, and conduction do not involve chemical changes 

 in nerves, it may be concluded that neither do they in 

 any other tissues. Thus, on account of the absence 

 of evidence of any chemical changes accompanying irrita- 

 bility in nerves, we have gradually drifted away from the 

 notion that the fundamental condition for protoplasmic 

 activity is chemical. 



