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 excit- 

 able 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 independent 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. 214), 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. 



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 catelectro- 

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

 core model' (' Kernleiter ') (Fig. 126) 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 polarisa- 

 tion. 



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 propa- 

 gated along the cable at a certain finite velocity which can be calcu- 

 lated 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 



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