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 init. 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. 
