72 A CHEMICAL SIGN OF LIFE 



Summary. The main points brought out by the 

 study of narcosis are: (i) 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 



