72 A CHEMICAL SIGN OF LIFE 
Summary.—The main points brought out by the 
study of narcosis are: (1) carbon dioxide production is 
greatly diminished when the nerve is narcotized either 
by chloral hydrate or by ethyl urethane in concentra- 
tions which produce a reversible loss of excitability; 
(2) with a weak concentration of these narcotics, at the 
beginning, the carbon dioxide production is increased, 
but later is diminished. This is in accord with the facts 
that these concentrations primarily stimulate, or in- 
crease, the irritability of the nerve for a time. The 
conclusion drawn from these facts is that metabolism in 
the nerve is interfered with by any agency which inter- 
feres with the excitability of the nerve. Excitability 
and resting respiration go hand in hand. 
The direction of the nerve impulse and the metabolic 
gradient—Although it has been established that an 
excitation wave travels in both directions from the point 
of the stimulus and that this wave is in all probability 
identical with the nerve impulse, yet in the normal con- 
dition in the body one fiber is supposed to conduct 
the impulse in one direction only. Based on this 
difference in the direction of the conduction, one set of 
the nerve trunks is called efferent and the other afferent, 
according as they conduct from or toward the central 
nervous system. That there is a very interesting rela- 
tion between the direction in which the impulse normally 
goes and the rate of metabolism at different parts of the 
nerve will be set forth in the following paragraphs. 
Efferent fibers —If we take the bundle of nerves 
between the second and third joints of the claw of the 
spider crab and cut it at the middle, the two halves being 
of about equal weight, and place each in a chamber of the 
