SECTION IX 

 THE NATURE OF THE EXCITATORY PROCESS 



UNDER this heading we have really two questions to discuss, namely, (a) the 

 nature of the change excited at the stimulated spot in an excitable tissue, 

 and (6) the propagation of the excitatory change away from the excited spot, 

 e.g. down a nerve fibre. That these two phenomena are more or less in- 

 dependent and may be dealt with separately is shown by the result of passing 

 a constant current through a parallel-fibred muscle, such as the sartorius. 

 In this case, as we have seen (p. 192), at make of the current an excitatory 

 change occurs at the cathode and is transmitted throughout the whole 

 length of the muscle, giving rise to a twitch of the muscle. During the 

 passage of the current there is still an excitatory change at the cathode, 

 but limited to a region within one or two millimetres of the cathode. 

 I An attempt has been made by Boruttau and other physiologists to 

 explain the nerve process, not as a wave of electrical change affecting the 

 substance of the axis cylinder itself, but as a propagated catelectrotonic 

 current. This observer found that, by working with a ' platinum core 

 model ' (' Kernleiter ') (Fig. 125) of considerable length, the catelectrotonic 

 current was developed at one end of the model some appreciable time after 

 a current had been sent in at the other end, thus resembling a current of 

 action. It is however impossible to explain all the electrical phenomena 

 of nerve as due simply to polarisation. We might go so far as to assume 

 that the excitatory effect at the cathode is due to negative polarisation, 

 and that excitation at break, i.e. at the anode, is caused by the sudden 

 coming into existence of a negative polarisation current ; but then it would 

 be difficult to understand how the excitation, so produced at the anode, should 

 give rise to a current so much exceeding the current which produced it that 

 it would appear in our external circuit as a current of positive polarisation. 

 The same objection would hold to the comparison of a nerve-fibre with 

 a submarine cable. An electric disturbance produced at any part of a cable 

 (i.e. a conducting wire in an insulating sheath) is propagated along the 

 cable at a certain finite velocity which can be calculated when we know 

 the conductivity of the core, the capacity of the cable, and the di-electric 

 constant of the sheath. In all these cases there must be a decrement of 

 the change as it is transmitted away from its seat of origin, a decrement for 

 the existence of which there is no evidence in a nerve fibre or other excitable 

 tissue.* Moreover the phenomenon of propagation of an excitatory process 



* It might be urged, on the other hand, that one would not expect to find any 

 appreciable decrement in a cable only 1 to 3 inches long. 



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