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 
