SUMMARY OF THE CHAPTER 151 



Let the current be passing through the fish from the anterior to 

 the posterior electrode. At the tail end we get no excitation ; 

 and, as we pass forward, the body remains quiet until, pass- 

 ing the region of the medulla oblongata, an excitation appears. 

 If the operation is repeated with a reverse current, we get 

 excitation behind the head and quiet on the head. Secondly, 

 if a frog larva be cut in two transversely at the root of the 

 tail, the head end is irritated only by a caudad-flowing cur- 

 rent ; the tail end only by a cephalad-flowing current ; while 

 in both cases the opposite current quiets. (Of. also Ewald, 

 '94^ p. 162.) These observations were now made use of to 

 explain the opposite orientation of the tadpole in the presence 

 of weak and strong currents. Nagel assumed that the weak- 

 est currents can affect the brain only. Now if that current 

 runs caudad, it will strongly stimulate the body so that it turns 

 tail to the anode. The medium currents, however, stimulate 

 the whole dorsal nerve, but the spinal cord to a preponderating 

 degree, so that a cephalad-passing current irritates more than a 

 caudad current, and the animal will turn tail to the kathode. 

 Thus weaker or stronger current will determine — or -|- 

 electrotaxis. 



Stjmmaey op the Chapter 



Electricity affects protoplasm in two principal ways : first, 

 by causing contraction ; second, by determining orientation. 

 We can distinguish two principal types of contraction phe- 

 nomena and, corresponding to and dependent upon these, two 

 types of orientation phenomena. The first type of contraction 

 is that which is produced, upon making the constant current, 

 chiefly at the anode ; the second is produced chiefly at the 

 kathode. The corresponding orientation or migration types 

 are, facing the kathode and facing the anode. Since the orien- 

 tation phenomena are dependent upon the contraction phe- 

 nomena, the most important causes to be investigated are, first, 

 that of contraction, and second, that of the difference in type 

 of contraction exhibited by different organisms. The funda- 

 mental teaching of this chapter is that the electric current acts 

 as a stimulus upon protoplasm, and may determine the charac- 

 ter of its activities. 



