ORIENTATION AND FUNCTION 163 



the current .is exactly right. The back is convex, the 

 ventral side concave. But if the current passes from 

 tail to head, the back becomes entirely straight, the 

 extensors being contracted to the utmost limit. 1 The 

 dorsal side cannot become concave because the exo- 

 skeleton of the crayfish does not allow it. Thus in 

 the body of the crayfish the motor elements of the 

 extensors and flexors which are affected by the cur- 

 rent must have the same orientation as in the body of 

 Vertebrates. This holds good not only for the flexors 

 and extensors of the body but generally, as we shall 

 at once see. 



We have already mentioned the fact that a con- 

 stant current passing through Amblystoma larvae in 

 the longitudinal direction affects not only the tension 

 of the flexors and extensors of the body, but also 

 the muscles of the extremities. The tension too is 

 changed in such a way, as has been already intimated, 

 that it renders movement toward the anode easy, 

 movement toward the kathode difficult. If, for in- 

 stance, the current passes through the animal from 

 head to tail the hind-le^s are braced backward and 



fj 



the position of the fore-legs is changed correspond- 

 ingly, so that the progressive movement of the ani- 

 mal is made easy, the backward movement dif- 

 ficult. If, however, the current passes from tail to 

 head the hind-legs are braced forward and the posi- 

 tion of the fore-legs is changed correspondingly ; the 



1 It is necessary that in this experiment the intensity of the current be 

 increased very slowly. 



