CHAPTEE XV 



ELECTROTONUS 



WHEN a constant current is thrown into a nerve, there is an excita- 

 tion which leads to a nervous impulse, and this produces a contraction 

 of the muscle at the end of the nerve. Similarly, there is another 

 contraction when the current is taken out. While the current is 

 flowing through the nerve, the muscle is quiescent. But while the 

 current is flowing there are changes in the nerve, both as regards its 

 electrical condition and its excitability. These changes are summed 

 up in the expression electrotonus. 



In the investigation of this subject the instruments employed are 

 the same as those already studied, with the addition of two others 

 that it will be convenient to describe before passing on to the study 

 of electrotonus itself. These are the reverser or commutator, and 

 the rheochord. 



Pohl's commutator is the form of reverser generally employed. It 

 consists of a block of ebonite provided with six pools of mercury, 



FIG. 165. Pohl's Commutator, with cross wires. (After Waller.) 



each of which is provided with a binding screw. The corner pools 

 are connected by diagonal cross wires, and by a cradle consisting of 

 an insulating handle fixed to two arcs of copper wire which can be 

 tilted so that the two middle pools can be brought into communication 

 with either of the two lateral pairs of pools. Fig. 165 shows how, by 



