842 ELECTRO-PHYSIOLOGY 



Whether the electrical organ is the homologue of muscle or of nerve- 

 ending, or whether it is related to either, has been much discussed. 

 Our surest guide in a question of this sort is the study of development; 

 and researches along this line have shown that there are two kinds oi 

 electrical organ, one being modified muscle (as in Gymnotus, Torpedo, 

 and the skate) ; the other transformed skin-glands (as in Malapterurus) . 

 The scanty blood-supply of the electrical organs in comparison with that 

 of muscle is noteworthy. In no case do bloodvessels enter the substance 

 of the plates. 



PRACTICAL EXERCISES ON CHAPTER XV. 



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 tripcd so that the legs come 

 in contact with one of the iron feet. Whenever this happens, the 

 circuit for the current set up by the contact of the copper and iron is 

 completed, the nerves are stimulated, and the muscles contract (p, 822). 



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, stimulates 

 the nerve, and causes contraction of the muscle (p. 822). 



3. 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. 305, p, 833). Place under the nerve near its 

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

 tudinal 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, 283, p. 808). On 

 closing or opening the key both muscles contract. Arrange the induc- 

 tion 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. 



4. Demarcation Current and Current of Action with Capillary Elec- 

 trometer. (a) Study the construction of the capillary electrometer 

 (Fig. 235, p. 729). Raise the glass reservoir by the rack and pinion 

 screw, so as to bring the meniscus of the mercury into the field. Place 

 two moistened ringers on the binding-screws of the electrometer, open 

 the small key connecting them, and notice that the mercury moves, a 

 difference of potential between the two binding-screws being caused 

 by the moistened fingers. 



(b) Demarcation Current. Set up a pair of unpolarizable electrodes 

 (Fig. 238, p. 731). Fill the glass tubes about one-third full of kaolin 

 mixed with physiological salt solution till it can be easily moulded. 

 To do this, make a piece of the clay into a little roll, which will slip down 

 the tube. Then with a match push it down until it forms a firm plug. 

 Next put some saturated zinc sulphate solution in the tubes, above the 

 clay, with a fine-pointed pipette. Fasten the tubes in the holder fixed 

 in the moist chamber (Fig. 322). Now amalgamate the small pieces of 

 zinc wire (p. 197) which are to be connected with the binding-screws of 

 the chamber. (Or use Porter's ' boot ' electrodes. These are made of 

 unglazed potter's clay. In use the leg of the boot is half-filled with 



