91 



lay it on a glass plate. Dissect out a long piece of the left 

 sciatic nerve. Remove and arrange it in such a way upon a 

 block of paraffin that one centimeter of it overlaps a corre- 

 sponding length of the right sciatic, fig. 21. 



Stimulate the left nerve with a single induction shock ; 

 the muscle contracts. Stimulate with the interrupted current ; 

 the muscle is thrown into tetanus. Stimulate also with the 

 constant current and compare effects. If properly conducted 

 this experiment will also show that a nerve impulse can pass 

 in both directions. 



Stimulate the left muscle directly by applying the elec- 

 trode to it and note any effect upon the right muscle. 



Ligate the left sciatic nerve between the electrodes and 

 the right nerve ; stimulate again. The muscle does not con- 

 tract. In the former case, therefore, its contraction was not 

 due to an escape of the stimulating current. 



328. Secondary Contraction From the Heart. Ex- 

 cise the heart ; lay the nerve of a fresh nerve-muscle prepara- 

 tion upon it as per diagram, fig. 22. The muscle contracts at 

 each beat of the heart, being excited by the electrical current 

 which accompanies each beat. 



.^/y< 



Fig. 22. 

 Fig. 22. Secondary contraction from the heart. 



Crush the apex of the ventricle with the forceps and ar- 

 range the nerve so that its cut end will come in contact with 

 the injured portion of the heart. Note the result. 



329. Paradoxicai, Contraction. Arrange the battery, 

 and key for a constant current. 



Pith a frog and expose the sciatic nerve down to the knee 

 Trace out the two branches into which it divides, fig. 23. Cut 

 off one of these branches as near as possible to the knee and 

 stimulate near its cut end. The muscles, supplied by the other 

 branch of the nerve, contract. Try mechanical or chemical 

 stimulation of the same branch. What is the result ? 



