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 resultini; 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 pr()i)agatecl 

 excitation follows. The differences between the i)hysical 

 conditions and manifestations of the two j)r()cesses, 

 local change and propagated disturbance, and thi- 

 differences in their temperature-coefficients 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 j)olarizalion 

 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 Xernst's simj^lc theor>' 

 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 i)oints out 

 that in any case of electrical stimulation ihc 

 concentration-changes at tico semi-permeal)le surfaces 



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



