CHAPTER XIII. 



1. The nerve-current ; 2. Negative variation of the nerve-current ; 

 3. Duplex transmission in the nerve ; 4. Rate of propagation 

 of negative variation; 5. Electrotonus ; 6. Electric tissue of 

 electric fishes ; 7. Electric action in plants. 



1. In addition to the many points of similarity 

 between muscles and nerves exhibited in their be- 

 haviour when irritated, it cannot escape notice that 

 the nerves also exhibit electric phenomena, and that 

 they do this in exactly the same way as does muscle. 

 Nerves being formed of separate parallel fibres, these 

 phenomena are exactly analogous to those in a regular 

 muscle-prism ; only that in a cross-section of a nerve, 

 on account of its small extent, differences of tension 

 cannot be shown at the various points, and the cross- 

 section must be regarded as a single point. 



In an extracted piece of nerve all the points on 

 the upper surface, that is, on the longitudinal section, 

 are as a fact positive, in distinction from those on the 

 cross-section, which are all of one kind. On the longi- 

 tudinal section the greatest positive tension is always 

 in the centre, and the tension decreases toward the 

 cross-sections, just as in the muscle-prism, at first 

 slowly, afterwards more abruptly, as shown in fig. 59. 



Because of the small diameter of the nerve-trunks, 

 distinction cannot, of course, be drawn between straight 



