CHEMICAL SIGNS OF IRRITABILITY 35 



plasmic respiration and growth have already been 

 pointed out. 1 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 



1 Mathews, Textbook of Physiological Chemistry, 1915, p. 67. 



