Carbon Dioxide From Nerve Fibres 133 



tability diminishes. These facts prove directly that the nerve con- 

 tinuously undergoes chemical changes, and that nervous excitability 

 is directly connected with a chemical phenomenon. There is still 

 another question left, namely, Is there any direct relation between 

 excitability and tissue respiration? To put this question more directly, 

 we may ask: Does excitability depend on the respiratory process in 

 the protoplasm? To answer these questions we must refer to two facts; 

 namely the direct relat on between the rate of respiratory activity and 

 the decrease of excitability; secondly, the influence of reagents on 

 CO 2 production and their effects on the state of excitability. 



By the studies of C0 2 production by Fletcher 48 lactic acid forma- 

 tion by Fletcher and Hopkins, 49 and heat evolution by A. V. Hill, 50 

 it has been established that in isolated muscle, respiratory processes 

 decrease when irritability diminishes. In the case of the nerve, 

 as shown in Table 3, C0 2 production reaches this minimum when 

 excitability approaches zero. These relations, however, do not show 

 conclusively that the protoplasmic irritability depends on respiratory 

 activity, for it is quite probable that the dying nerve may alter its 

 physical condition as well, which according to the physical school, 

 may consequently alter the state of excitability. 



That irritability is independent of the respiratory processes has 

 been hitherto successfully contended in the case of the dry seed. The 

 works of Horace Brown, Thisel ton-Dyer 51 and others indicate that 

 the dry seed can be kept alive at the conditions where no ordinary 

 gaseous exchanges are possible. It is argued, therefore, that life is 

 possible without any metabolic activity. 52 While a definite poten- 

 tiality for irritability may exist without any metabolic activity, yet 

 that the irritability can persist without respiratory activity, or vice 

 versa, is a matter by no means settled. In the case of ordinary 

 air-dry seed, Waller could demonstrate the response of electrical 

 changes when stimulated although the detection of CO 2 was impossi- 



48 FLETCHER: loc. cit. 



49 FLETCHER and HOPKINS: loc. cit. 



50 A. V. HILL: loc. cit. 



51 THISELTON-DYER: Proceedings of the Royal Society, 1897, Ixii, p. 160; 



ibid., Ixv, p. 361. 



52 I am indebted to Professor Crocker for his kind suggestion as to botanical 



literature. 



