136 THE PHYSIOLOGY OF NERVE 



in medullated nerves, it is said to be more intense in non-medullated 

 nerves. Moreover, its strength diminishes very rapidly and especially 

 in the nerves of warm-blooded animals, but the previous difference in 

 potential may again be established by making a new section next to 

 the first. Injured nerves, therefore, behave in the same manner as 

 injured or degenerating muscle. In either tissue the current flows 



I&fB 



Fig. 79. — Current of Action in Nerve. 

 To begin with the nerve shows the current of injury indicated by the arrows (as 

 in Fig. 78). When stimulated at >S a negativity passes along the nerve which, on 

 reaching Pole A, causes a partial reversal of the current of injury, indicated by the 

 needle. 



through the galvanometer from Ihe non-injured to the injured portion, 

 and inside the nerve from the injured to the non-injured. The latter 

 we call the axial current. An interesting modification of this axial 

 current^ has been observed in nerves normally possessing a mixed 

 direction of conduction. Thus, it has been found that the two cross- 

 sections of a nerve are equipotential only in a mixed nerve, while 



% 



JL 



Fig. 80. — Schema to indicate the procedure used to prove the diphasic character of 

 the action current. The isoelectric condition obtained to begin with is destroyed as soon 

 as the wave of negativity arrives at lead A. 



nerves composed either of afferent or efferent fibers, present distinct 

 differences in potential. In an afferent nerve, the central cross-sec-^ 

 tion is galvanometrically negative to the peripheral, while in an effer- 

 ent one it is positive to the peripheral. Thus, excised segments of 

 nerve always exhibit an axial stream in a direction opposite to that 

 of their normal conduction, namely, descending in afferent nerves and 

 ascending in efferent nerves. 



^ DuBois-Reymond, Unters. iiber tier. Elektrizitat, ii, 252; also see: Weiss,, 

 Pfliiger's Archiv, cviii, 1905, 416. 



