io6 A CHEMICAL SIGN OF LIFE 



bicycle running rapidly will not fall down. The process 

 of standing up may be compared to the vital process 

 in the resting state. The degree of irritability will be 

 measured by the force necessary to change the angle it 

 makes with the ground. Perhaps the amount of life may 

 be compared to the angle the bicycle makes with the 

 ground. The metabolic activity is the force which 

 moves the wheel. The locomotion of the wheel is the 

 functional activity. There is, however, this difference- 

 the faster the bicycle moves the more stable it is, whereas 

 the faster the respiration drives the less stable is the 

 irritability of the tissues. Our simile breaks down here. 

 And indeed it is but a poor picture, of not much value. 

 But whatever view we may take of the matter, we may 

 at least be sure of this much: that chemical change is in- 

 volved in irritability. The transmission of a nerve 

 impulse involves material decomposition in the fiber. 

 The impulse may be nothing else than the increased 

 metabolism itself. The nerve impulse is a very real 

 thing, and it has a material basis which we may hope to 

 discover. So far we have found two facts about it: 

 first, it liberates carbon dioxide as it passes over the 

 fiber; and, second, it depends on the nerve fiber having 

 been previously oxidized or exposed to oxygen. Evi- 

 dently combustion is involved in the process somewhere, 

 but it appears at present more probable that it is involved 

 in the creation of the irritable substance rather than in 

 the very act of excitation itself. In other words, the 

 oxidation is part of the process of repair or the recovery 

 of the tissue the process by which the state of irrita- 

 bility is maintained and not the process of transmission 

 of the impulse itself. 



