STIMULATION AND TRANSMISSION 293 



effect or stimulation-process proper. The former is, 

 or may be, a purely physical change; in electrical 

 stimulation its essential feature is apparently a change of 

 polarization resulting from changes of ionic concentration 

 at the cell surface. This change does not initiate a 

 propagated effect unless it exceeds a certain critical 

 limit, and unless the state of the tissue is favorable; 

 thus in an anaesthetized tissue the local change of 

 polarization is produced by a current, but no propagated 

 excitation follows. The differences between the physical 

 conditions and manifestations of the two processes, 

 local change and propagated disturbance, and the 

 differences in their temperature-coefffcients show that 

 the propagated process is more complex than the initia- 

 tory local process and includes chemical or metabolic 

 factors among its chief components. 



GENERAL NATURE OF STIMULATION CHANGES 



The factors determining the characteristic chronaxie 

 of a tissue would thus appear to be largely factors 

 determining the rate at which the critical polarization 

 change occurs in the irritable elements. This rate 

 depends on the rate of movement of ions and also on the 

 ' special structural conditions within the tissue. An impor- 

 tant advance in the theory of the local change has been 

 made by Hill,^ who has modified Nernst's simple theory 

 and brought it into closer conformity both with the 

 facts of organic structure and with the actual behavior 

 of the tissue in electrical stimulation. Hill points out 

 that in any case of electrical stimulation the 

 concentration-changes at two semi-permeable surfaces 



^ A. V. Hill, Journal of Physiology y XL (1910), 190. 



