CONCLUSIONS 97 



There is no functional activity without the simultaneous 

 consumption of the nervous material. In the nerve, 

 then, irritability can be measured by the increased 

 metabolism which occurs on stimulation. If the irrita- 

 bility is high, the carbon dioxide increment is also large, 

 and vice versa. The response to stimulation is the sign 

 of irritability. We measure this response by measuring 

 the simultaneous output of carbon dioxide, which must 

 be the sign of that metabolic activity in virtue of which 

 the function is performed. 



In chapter iv we demonstrated further the importance 

 of the metabolic changes for the functional activities of 

 the nerve, and we hinted that the real mechanism which 

 makes the nerve able to perform its function must be 

 the chemical changes which go on in the resting con- 

 dition of the nerve, and which must determine not only 

 the degree of excitability of the nerve the direction of 

 the nerve impulse but also how fast this transmission 

 travels. We found, for example, that the part of the 

 nerve from which the nerve impulse normally comes 

 always produces more carbon dioxide than the part 

 toward which it is going. It is well known that nerves 

 are more excitable in the parts which normally originate 

 the impulse and that the excitability decreases down 

 the fiber. Thus there is a parallelism between the 

 degree of excitability and the amount of carbon dioxide 

 produced in different parts of the same fiber, a parallelism 

 the profound importance of which is not easily over- 

 looked. For nerves are thus shown to have in them a 

 metabolic gradient. They are, as it were, polarized 

 metabolically, and thus we have for the first time the 

 explanation of the electrical current which has been 



