PR A CTICA L EXER CISE S 



651 



PRACTICAL EXERCISES ON CHAPTER XL 



1. Galvani's Experiment. Pith a frog (brain and cord). Cut 

 through the backbone above the urostyle, and clear away the anterior 

 portion of the body and the viscera. Pass a copper hook beneath 

 the two sciatic plexuses, and hang the legs by the hook on an iron 

 tripod. If the tripod has been painted, the paint must be scraped 

 away where the hook is in contact with it. Now tilt the tripod so 

 that the legs come in contact with one of the iron feet. Whenever 

 this happens, the current set up by the contact of the copper and 

 iron is completed, the nerves are stimulated, and the muscles 

 contract (p. 628). 



2. Make a muscle-nerve preparation from the same frog. Crush 

 the muscle near the tendo Achillis, so as to cause a strong demarcation 

 current. Cut off the end of the sciatic nerve. Then lift the nerve 

 with a small brush or thin glass rod, and let its cross-section fall on 

 or near the injured part of the muscle. Every time the nerve touches 

 the muscle a part of the demarcation current passes through it, stimu- 

 lates the nerve, and causes contraction ot the muscle (p. 628). 



3. Make a muscle-nerve preparation. Lay it on a glass plate A, 

 supported on a block of wood. 



Snip off the end of the nerve N, and 

 arrange the cut surface on a pad of 

 kaolin B, moistened with normal 

 saline. Another pad B' is placed 

 under the nerve a little way from 

 its cut end. Both pads project 

 down over the edge of the glass 

 plate. A watch-glass C filled with 

 normal saline solution is lifted up 

 below the projecting ends till they 

 are immersed. Whenever this 

 happens, a circuit is completed for 

 the demarcation current of the 

 nerve itself, by which it is stimu- 

 lated, and the muscle M contracts 

 (Fig. 220). 



4. Secondary Contraction. Make two muscle-nerve preparations. 

 Lay the cross-section of one of the sciatic nerves on the muscle of 

 the other preparation (Fig. 2 12, p. 644). Place under the nerve near 

 its cut end a small piece of glazed paper or of glass rod, and let the 

 longitudinal surface of the nerve come in contact with the muscle 

 beyond this. Lay the nerve of the other preparation on electrodes 

 connected with an induction machine arranged for single shocks, with 

 a Daniell cell and a spring key in the primary circuit (Fig. 189). 

 On closing or opening the key both muscles contract. Arrange the 

 induction machine for an interrupted current. When it is thrown 

 into one nerve, both muscles are tetanized ; the nerve lying on the 

 muscle whose nerve is directly stimulated is excited by the action 

 current of the muscle. 



FIG. 220. STIMULATION OF A NERVE 

 BY ITS OWN DEMARCATION CUR- 

 RENT. 



