738 A MANUAL OF PHYSIOLOGY 



shock is about o^j second. The skate must be included in the" list 

 of electric fishes. Although its organ is relatively small, and its 

 electromotive force relatively feeble, yet it is in all respects a com- 

 plete electrical organ. It is situated on either side of the vertebral 

 column in the tail. The plates or discs are placed transversely and 

 in vertical planes. The nerves enter their anterior surfaces ; the 

 shock passes in the organ from anterior to posterior end. Gotch 

 and Sanderson have estimated the maximum electromotive force of 

 a length of i cm. of the electrical organ of the skate at about half a 

 volt. 



Whether the electrical organ is the homologue of muscle or of 

 nerve-ending, or whether it is related to either, has been much dis- 

 cussed. 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 of 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 XI. 



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 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. 717). 



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

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

 tion 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. 717). 



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. 279, p. 729). 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. 258, p. 703). On closing or opening the key both muscles con- 

 tract. 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. 



