210 PHYSIOLOGY CHAP. 



lies nearer the peripheral section in the afferent nerve, nearer the 

 central section in the efferent nerve. 



This phenomenon, discovered by Du Bois-Eeymond and 

 confirmed by Fredericq, Mendelssohn, and others, indicates that 

 efferent nerves are traversed by an ascending, afferent nerves 

 by a descending axial current. This is the only objective 

 difference known at present between the two categories of nerves, 

 which are alike in structure and in their capacity for conducting 

 in both directions. 



According to the latest work of Weiss (1904), the potential 

 difference between two cross-sections of nerve the axial current 

 is due solely to an anatomical cause, the unequal distribution 

 of the connective tissue. The more connective tissue present, 

 the less the potential that can be led off", owing to the resulting 

 short circuit. The contrary direction of the axial current in 

 efferent and afferent nerves might also be the result of unequal 

 arrangement of the connective tissue. 



/ 



Centrifugal nerve. Centripetal nerve. 



Fio. 135. Diagram of axial currents in centripetal and centrifugal nerves. (Du Bois-Eeymond.) 

 c, central end ; ;<, peripheral end ; e, physiological equator. 



The discovery of the current of rest or demarcation current 

 was immediately followed by that of the current of action, i.e. 

 the electromotive phenomena produced by stimulating a nerve, 

 which correspond perfectly with those already noticed for muscle. 

 In nerve as in muscle the current of action is manifested as the 

 negative variation of the demarcation current. If a current from 

 one end of the divided sciatic of the frog is led off to the galvano- 

 meter, it is reduced or abolished on exciting the other end with 

 a tetanising current. When stimulation ceases, recovery of the 

 original state is manifested. 



Du Bois-Keymond's phenomenon of the negative variation can 

 also be demonstrated with chemical stimuli (Griitzner), mechanical 

 stimuli (Hering), and physiological stimuli (Gotch and Horsley) 

 for both afferent and efferent nerves. It is seen in afferent 

 nerves when the peripheral stump of the dorsal root of a 

 mammalian spinal nerve is connected with the galvanometer, 

 either the peripheral nerve or the sensory nerve-endings of the 

 skin being excited at a distance. It is seen in efferent nerves on 

 leading off the central end of the ventral root, or sciatic, to the 

 galvanometer, and exciting the ganglion cells of the cord or the 



