CHEMICAL SIGNS OF IRRITABILITY 35 
plasmic respiration and growth have already been 
pointed out.t. It is unnecessary to go into this more 
fully than to call attention to the facts that linseed oil 
takes up oxygen, that it gives off carbon dioxide, that it 
is stimulated by light, that it undergoes also other 
phases of metabolism common to living matter, and 
that, very singularly, it exhibits many of the phenomena 
of memory, learning, and forgetting. It is striking, 
too, that this respiration is of an autocatalytic nature— 
that is, it becomes more rapid as it progresses and in this 
respect resembles the psychic phenomena of memory and 
learning. Thus we see that respiration alone cannot 
be taken as a criterion of life; and, furthermore, that 
even the characteristic features of protoplasmic respira- 
tion itself cannot be said to be peculiar to living things. 
A more certain criterion of life is the increase of respira- 
tion on stimulation. 
It is well known that contracting muscle produces 
more carbon dioxide than resting muscle. We breathe 
faster when we run. We can measure the irritability of 
the muscle by its increased metabolism occurring on 
stimulation. Is it possible to increase the metabolism 
of the nerve also by stimulation? Can the nerve, one 
of the most irritable tissues of all, perform its function 
without consuming any material? Is the nerve impulse 
something similar to an electrical current passing 
through a rather imperfect conductor? How is the 
electromotive force created in the nerve fiber when the 
impulse passes through it? Is it simply the equiva- 
lent of the energy we put in at the initial stimulation ? 
These questions cannot be considered unless we first 
t Mathews, Textbook of Physiological Chemistry, 1915, p. 67. 
