GENERAL NERfE PHYSIOLOGY 217 



reaches a, and an electric current passes from b to a through Z. 

 When the impulse has passed a and is carried to , a current passes 

 from a to b through Z. These currents are called action currents. 

 Their rapidity is so great and they follow each other so closely that 

 they cannot be demonstrated by an ordinary galvanometer. They 

 can be demonstrated by a very sensitive electrometer (capillary 

 electrometer) or by a special apparatus in which the action upon 

 a magnetic needle is intensified by a series of action currents 

 moving in the same direction and rapidly following each other 



JL_ 

 FIG. 17. 



through the galvanometer (Bernstein's repeating differential 

 rheotome). 



Cut a nerve across, place one electrode of the galvanometer on 

 the transverse section and the other on the longitudinal surface. 

 A current will pass through the galvanometer from the longitudinal 

 to the transverse section (current of rest). At the cut surface the 

 nerve immediately begins to die, and this is accompanied by 

 processes which make that part of the nerve negatively electrical 

 to the sound part. If in such a case a certain point in the longi- 

 tudinal surface be stimulated, the intensity of the current of rest 

 is decreased, that is, it undergoes a negative variation. 



Aside from these electrical changes, the nerve impulse 

 can only be detected by the effects which it has upon the 

 end-organ (muscular contraction in the motor nerves, sensa- 

 tion in the sensory nerves). 



2. Laws of conductivity of nerves. 



(a) Isolated conduction. In a nerve trunk composed of 

 many fibres the impulse does not pass from one fibre to 

 another. 



(b) Double conduction. A nerve artificially stimulated at 

 a certain point conducts the impulse not only in the direc- 

 tion in which it is conducted under physiological conditions, 

 but also in the opposite direction. 



The nerve supplying the gracilis muscle of the frog divides into 

 two branches, of which the one supplies the upper and the other 

 the lower half of the muscle. At the forking of the nerve, the 



