208 PHYSIOLOGY CHAP. 



in any part of a nerve is accomplished instantaneously at the 

 expense of the next part, and that upon this the propagation of 

 the excitatory impulse depends. 



An indirect proof of this theory is afforded by the study of 

 the electrical phenomena exhibited by nerve in the state of rest 

 and of activity, which need only a brief description, since they are 

 almost exactly identical with those already discussed for muscle 

 (vide Chapter' I., sec. XL, p. 68). 



The discovery of the so-called current of rest in nerve was 

 made by du Bois-Eeymond (1845). Any bit of nerve cut out 

 of the body presents approximately the same electromotive 

 phenomena as muscle, and these may be summed up as follows : 



(a) Two symmetrical points on the longitudinal surface and of 

 the two cross-sections of a nerve are as a rule iso-electric, i.e. 

 equipotential. 



(&) Two points at different distances from the sections show a 



FIG. 134. Diagram of demarcation currents in a length of mixed nerve excised from the animal. 

 Direction of currents indicated by arrows ; e, physiological equator at the centre of the bit 

 of nerve. 



difference in potential, in the sense that the point nearest the 

 cross - section is electrically negative, on the galvanometer, as 

 compared with the other point. 



(c) Generally speaking, the surface of a transverse section is 

 negative to the natural or longitudinal surface, and the greatest 

 difference in potential, i.e. the maximum deflection of the galvano- 

 meter needle, is obtained on placing one unpolarisable electrode 

 on the cut surface and the other on the middle of the longitudinal 

 surface. 



The diagram in Fig. 134 is a representation of these pheno- 

 mena. They are all comprised under the general law that in 

 excised nerve the longitudinal surface represents the positive pole 

 or anode, and the transverse surface the negative pole or kathode. 



The currents that can be led off to a galvanometer from an 

 artificial cross-section and from any given point of the natural 

 longitudinal surface of a nerve, decline rapidly, especially in 

 warm-blooded animals. In the frog's sciatic the value of the 

 current may fall by one-half in two to four hours, especially in 

 summer. But the difference of potential may increase again, and 

 the current may regain its original force, if a new section is made 



