518 NERVE, 



that during the flow of a current directed through the retina from the 

 layer of nerve cells to that of the rods and cones (anode on the eyelid), 

 the retinal excitation is such as to give the consciousness of bright 

 colour, the flow in the opposite direction is such as to give the con- 

 sciousness of a dark contrast colour. In such instances the visual 

 characters of the field must be taken as those associated with the 

 retinal excitation, the visual characters of the centre being due to 

 contrast. 



These results, although showing discrepancies in the hands of 

 different observers, may be taken as indicating, first, the existence of 

 both initial and prolonged closing and opening excitation effects ; and, 

 secondly, the existence of polar excitatory results of opposite kind, 

 but so linked that the production of the one is succeeded by that of the 

 other. They are thus in conformity with the views as to the nature 

 of polar excitation of nerve, which have been set forth in the preceding 

 pages. 



ELECTROMOTIVE CHANGES IN UNEXCITED NERVE. 



General characters of the nerve current. It was discovered in 

 1845 by du Bois-Eeyrnond that persistent electromotive changes might 

 be detected in all nerves. 1 The main features of such changes in an 

 excised nerve may be summed up as follows : 



1. Two symmetrical points on the longitudinal surface of a nerve are 

 equipotential. 



2. If one point is nearer a cross-section than the other, a difference 

 of potential is observed, the former being galvanometrically negative 

 to the latter. 



3. The maximum difference of potential is observed when one point 

 is the cross section itself and the other the longitudinal surface, the 

 tissue at the section being now markedly negative to that of the surface. 



The nerve fibres 

 of an excised nerve 

 are thus traversed 

 by axial currents, 



_ directed from the 



cross section along 

 the fibres ; and since 

 FIG. 269. the nerve is a moist 



conductor, a closed 



circuit for the spread of such currents is afforded by the adjacent 

 nerve fibres and by the moist surroundings. The electrical distribution 

 and direction of the currents are indicated in the annexed diagram 

 (Fig. 269), and since these are present in nerves which may be regarded, 

 at any rate for the present, as unexcited, they may be conveniently 

 described as currents of rest. 



The amount of the difference of potential varies with the degree 

 of approximation of the terminal contact to the cross section ; hence 

 the source of the electromotive change is undoubtedly to be found in 

 the difference between the molecular state of the nerve at any point 

 of its length, and that in the immediate vicinity of the cross section ; 

 the critical point is localised in the zone of demarcation between the 



1 du Bois-Reymond, "Untersuchungen," Bd. ii. (1), S. 289. 



