ORIENTATION AND FUNCTION 161 



muscles (the extensors of the spinal column) from the 

 passage of the current. On the other hand, if the 

 current goes through the animal in the direction from 

 tail to head both head (Fig. 38), and tail are raised. 

 The body becomes more concave on the dorsal side 

 and convex on the ventral side. The extensors of the 

 spinal column become more tense than the ventral 

 muscles. A pronounced opisthotonus exists. In 

 order to show the phenomenon clearly the animal 

 must be brought into the current gradually. If 

 we continue to raise the intensity of the current, 

 changes of position also take place in the legs. The 

 changes in the hind-legs are more easily described 

 than those in the fore-legs. If the current passes from 

 head to tail the hind-legs are braced backward (Fig. 

 37), making the forward movement (to the anode) 

 easier. If the current passes from tail to head the 

 hind-legs are braced forward (Fig. 38), making the 

 backward movement (to the anode) easier. How can 

 these phenomena be explained ? The current has two 

 kinds of effects. A conduction of the current takes 

 place through ions. Wherever the progress of ions is 

 blocked in the central nervous system, an increase in 

 their concentration will occur and this must be followed 

 by physical or chemical alterations of the colloids. The 

 progress of ions may be blocked by semipermeable 

 membranes at the external limit of neurons or some- 

 where inside the neurons. Wherever anions are 

 blocked different effects (anelectrotonus) will be pro- 

 duced than at places where the progress of kations is 



