SECONDARY CONTRACTION AND SECONDARY TETANUS. 195 



of wire composed of a copper and zinc wire soldered together. 

 Touch the nerves above with the copper (or zinc) end, and 

 the muscles below with the zinc (or copper), when con- 

 traction occurs at make, or break, or both. 



2. Contraction without Metals. Take a strong frog, make a 

 nerve-muscle preparation, leaving the leg attached to the femur, 

 and having the sciatic nerve as long as possible. Hold the 

 femur in one hand, lift the nerve on a camel's-hair pencil mois- 

 tened with normal saline, and allow it to fall upon the gastro- 

 cnemius, when the muscle will contract. Contraction occurs 

 because the nerve is suddenly stimulated, owing to the surface 

 of the muscle having different potentials. 



3. Secondary Contraction and Secondary Tetanus. 



(a.) Arrange the induction apparatus for single make and 

 break shocks. Pith a frog, dissect out two nerve-muscle pre- 

 parations, as in Fig. 85. 



(6.) Place the left sciatic nerve (A) over the right gastro- 

 cnemius (B), or thigh muscles, 

 and the right sciatic nerve over 

 the electrodes (E). 



(c.) Stimulate the nerve with 

 single induction shocks, and 

 note that the muscles of both B 

 and A contract. The contrac- 

 tion in A is called a secondary 

 contraction. 



(d.) Arrange the induction 

 machine for interrupted shocks, 

 and stimulate the nerve at E, 

 B is thrown into tetanus, and ,,. or , ~ 



so is A simultaneously. This Flg ' 8 %aS ^ 

 is secondary tetanus. This is a 



proof of the " action current " in muscle. The nerve of A 

 is stimulated by the variation of the muscle current during 

 the contraction of B. 



(e.) Ligature the nerve of A near the muscle, stimulate 

 the nerve of B ; there should be no contraction of A although 

 B contracts. 



