THE NATURE OF THE EXCITATORY PROCESS 323 



position relative to the membranes at which the changes of concentra- 

 tion are effective. From Lucas's experiments it would seem that the 

 changes of concentration occur in the immediate neighbourhood of 

 one of the membranes. Macdonald has brought forward evidence 

 that the passage of a current through a nerve involves the setting free 

 of certain inorganic ions. The subsidence of the excitatory state 

 depends on the rate of diffusion of ions. If, however, we compare the 

 rates of subsidence of the excitatory state in different tissues we find 

 much greater divergence than would be possible on the assumption 

 that the diffusion is one affecting inorganic ions. Thus between the 

 substance ft (the intermediate substance) of the frog's sartorius and 

 the ventricular muscle fibre of the same animal, the rate of subsidence 

 of the excitatory state changes in the ratio 4000 : 1. If the ions 

 concerned were simple ions, such as H', Ca*, Na% CK, &c., it would be 

 impossible to account for this wide variation, since their velocities differ 

 in the ratio of 10 : 1 at most. Moreover the effect of rise of temperature 

 on the rate of subsidence is greater than the effect of a similar rise on 

 ionic velocities. It is evident therefore that the theory is one for use 

 as a working hypothesis only. That excitation is associated with 

 accumulation of ions in the region of the exciting electrode, that the 

 subsidence of the excitatory state is due to disappearance by diffusion 

 or otherwise of these ions, there can be little doubt. But the questions 

 as to the nature of these ions, and their relation to the colloidal con- 

 stituents of the excitatory tissue, or to other possible substances, 

 changes in which may form an integral part in the excitatory state, 

 must be left for future investigation. 



