THE NATURE OF THE EXCITATORY PROCESS 321 



may travel the whole length of the nerve. Propagation would thus 

 involve the successive setting up of an excitatory process all along 

 the nerve or excitable tissue, though it is difficult to see why on this 

 theory every excitatory state should not give rise to a propagated 

 change. 



We are as yet a long way from a comprehension of the changes 

 involved in the process of excitation, though we are able to form some 

 idea of many of the factors which must be involved. Any theory 

 of the excitatory process must take into account the following 

 phenomena : 



(1) The excitatory state is attended with an electrical change of 

 such a nature that the excited spot is negative to adjacent unexcited 

 spots. This electrical change rises rapidly to a maximum and dies 

 away more slowly, the rate of its rise, and still more of its subsidence, 

 varying largely according to the nature of the tissue under investiga- 

 tion. 



(2) The excitatory change is aroused only at the poles of a current 

 passing through the tissue, i.e. at those places where polarisation can 

 occur in consequence of the electrical movement of ions. 



(3) Excitation only occurs at the kathode at make of the current, 

 and only occurs if the current attains a sufficient strength within 

 a certain period of time, the relation of strength of current to rate of 

 change varying in different tissues. 



(4) All living tissues are made up of colloids, divided into com- 

 partments by membranes of various permeabilities and permeated 

 with salts and other electrolytes in solution. 



Disregarding for the moment all considerations of structure, it is 

 possible to form a hypothesis of the nature of electrical excitation 

 which takes into account the facts just mentioned and enables us to 

 give a quantitative or mathematical expression to the factors involved. 

 An electrical current passing through a tissue containing membranes, 

 impermeable to the dissolved ions, will set up differences of concentra- 

 tions at and near the membranes. Nernst, on the supposition that 

 these differences of concentrations, when sufficiently large, would 

 cause an excitation, arrived at a formula connecting the lowest current 

 required to excite with its duration, and another formula connecting 

 the lowest amplitude of an electrical current with its frequency. 

 The mathematical investigation of the question has been continued 

 by A. V. Hill in conjunction with Keith Lucas. For this purpose 

 we may suppose that the excitable unit is represented by a cylindrical 

 space closed at its two ends by the membranes A and B (Fig. 132) and 

 filled with a solution of electrolytes. If a current be passed from 

 B to A the positively charged ions will move towards A and tend to 

 accumulate there. The accumulation of the ions near the membranes 



21 



