90 



and note that the muscles of both the rig-ht and left leg con- 

 tract. The contraction in the left leg is called a secondary 

 contraction. Repeat the experiment, using the constant cur- 

 rent. Note if there is any difference between the make and 

 break shocks. 



326. Secondary Tetanus. Prepare the induction ap- 

 paratus for interrupted shocks, and again stimulate the rig-ht 

 sciatic nerve. The right gastrocnemius muscle is thrown 

 into tetanus. The left g-astrocnemius is simultaneously 

 tetanized. This is known as a secondary tetanus, and is a 

 proof of the " action current " in muscle. The left sciatic is 

 stimulated by the variation of the muscle current during- the 

 contraction of the right g-astrocnemius. Ligate the left sci- 

 atic near its muscle ; stimulate the rigfht sciatic ; there should 

 be no contraction of the left gastrocnemius. 



Leaving the left sciatic in position, tie a lig-ature around 

 the right sciatic near its muscle and stimulate. Is there con- 

 traction in either preparation ? 



This experiment also shows that electricity, as such, is 

 not transmitted through the nerve as the thread of the lig-a- 

 ture is a conductor. The electricity serves merely as a stim- 

 ulus causing an impulse which can traverse the normal nerve 

 but cannot pass beyond a ligature or a crushed portion of the 

 nerve. 



electrode. . 



J'eri/e x.m.pu.Zse in, hoih direciions. 



Fig. 21 

 Fig. 21, «, nerve ; m, muscle. 



327. Secondary Contraction From Nerve. Make a 

 nerve-muscle preparation of the right hind leg of the frog and 



