chap, viii.] THE LAW OF CONTRACTION. 



89 



and note the result. Now reverse the commutator, 

 to get an upward current, and watch effects on 

 closing and opening with the same current strength. 

 Next, by moving the slider and taking out some plugs 

 of the rheocord, give a stronger current (medium), 

 and note results on closing and opening, first with a 

 downward, and then with an upward, current. Lastly, 

 interpose great resistance in the short circuit, to get 

 a strong current for the nerve, and observe the effects 

 of closing and opening with the current in different 

 directions. The results thus obtained should be 

 tabulated in the following way : 



i. 



LAW OF CONTRACTION. 

 II. 



III. 



where, at the head of each column, the strength of 

 the stream is indicated. Cl. means closing the circuit, 

 Op. means opening or breaking it, c. means contraction 

 of the muscle, and r. means rest, no contraction ; while 

 the direction of the current, J, , down the nerve, 

 or ^, up the nerve, is indicated at the side. Thus, 

 the first column would read : With a weak current, in 

 an upward direction, there was contraction on closing 

 the circuit, and rest on opening ; and with a downward 

 current of the same strength there was also con- 

 traction on closing and rest on opening. Experiments 

 should be made both with a quite fresh nerve and 

 with a nerve that has been allowed to lie for some 

 time after the death of the animal. This will show 

 that a weak current will give with a fresh nerve the 



