1912.] The Process of Excitation in Nerve and Muscle. 513- 



evidence is clear. Helmholtz* showed that in a nerve the passage of a 

 succession of propagated disturbances did not raise the temperature by as 

 much as 1/1000° C. Rollestonf lowered the possible change of temperature 

 to 1/5000° C, and within the present year Hill| has shown that the heat 

 liberated by the passage of a single propagated disturbance along a nerve 

 cannot be sufficient to raise the temperature of the nerve by more than about 

 a hundred millionth of a degree. This is the upper limit which the experi- 

 mental method can safely state not to be exceeded. Actually the most 

 sensitive instrument can detect no change at all. Such a finding means 

 that the large heat-liberation which occurs in muscle must be a part of the 

 contraction process and not of the propagated disturbance. The validity of 

 this inference is strengthened by another fact which Hill has demonstrated 

 by experiment, that in a given muscle under varying initial loads and with 

 varying degrees of contraction the heat liberated bears a constant ratio to the 

 tension developed. 



¥. The Importance of the Local Excitatory Process to Ifi/polhcses of the 

 Propagated Disturbance. 

 With this conclusion we are brought to the end of the first part of our 

 enquiry, the analysis of the phenomena concerned with the propagated 

 disturbance. We emerge from this analytical maze with some clear gain at 

 least. As far as hypotheses of propagation are concerned we can rule out of 

 consideration the process of contraction and the liberation of heat. The 

 refractory state, on the other hand, we have learned to regard as the 

 measure of a recovery process intimately concerned with the restoration of 

 equilibrium after that disturbance which is the basis of propagation. This 

 recovery process, with its characteristic progress in time,§ and its marked 

 prolongation by fall of temperature, will therefore have to be reckoned with 

 in any physico-chemical explanation of the process of propagation. But 

 perhaps the most significant step of the whole analysis has been that which 

 has failed to give positive results, the attempt to discover how the electric 

 response is related to the change by which propagation is effected. It is the 

 most significant because it brings home to us most clearly the incompleteness 

 of that knowledge on which any hypothesis of the propagated disturbance 

 must be based. But it is just here that we may look for help from another 

 result of our analysis, the recognition of the local excitatory process. This 



* Helmholtz, ' Arch. f. Anat. u. Physiol., 5 1848, p. 158. 

 t Eolleston, ' Journ. Physiol.,' 1890, vol. 11, p. 208. 

 \ Hill, ' Journ. Physiol.,' 1912, vol. 43, p. 433. 



§ Cf. Adrian and Keith Lucas, 'Journ. Physiol.,' 1912, vol. 44, p. 114, fig. 18. 



