650 A MANUAL OF PHYSIOLOGY 



electric fishes are far less excitable to electrical stimuli than the tissues 

 of other animals ; and this is found to be the case when their muscles 

 or nerves are tested with galvanic or induction currents. It requires 

 extremely strong currents to stimulate them ; and the electrical 

 nerves are more easily excited mechanically, as by ligaturing or 

 pinching, than electrically. In general, too, the shock is more 

 readily called forth by reflex mechanical stimulation of the skin than 

 by electrical stimulation. But that the organ itself is excitable by 

 electricity has been shown by Gotch. He proved that in Torpedo 

 a current passed in the normal direction of the shock is strengthened, 

 and a current passed in the opposite direction weakened, by the 

 development of an action current in the direction of the shock. And 

 indeed a single excitation of the electrical nerve is followed by a 

 series of electrical oscillations in the organ, which gradually die away. 

 The latent period of a single shock is about J D second. The skate 

 must now be added to the list of electric fishes. Although its organ 

 is relatively small, and its electromotive force relatively feeble, yet it is 

 in all respects a complete 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 electro- 

 motive 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 and Torpedo, and the skate), the other transformed skin 

 glands (as in Malapterurus). Curara does not affect the electrical 

 organ in Torpedo, although, as we have seen, it paralyzes motor 

 nerve-endings. 



Galvanotropism is the name given to a group of curious reactions 

 shown by many small animals, and particularly certain unicellular 

 organisms, when the liquid in which they swim is traversed by a con- 

 stant current. Some kinds of infusoria, e.g., Paramascium, travel in the 

 direction of the current towards the kathode and collect there, others 

 towards the anode. Animals higher in the scale, as small crabs, also 

 exhibit the phenomenon, and move towards the anode. Tadpoles 

 and young fish arrange themselves with their long axis in the direc- 

 tion of the current, and their heads turned to the anode. As to the 

 cause of the effects in those more highly organized animals, it has 

 been suggested that they take up the position in which they are least 

 stimulated, in order to escape disagreeable sensations excited by the 

 current. But the extensive studies of Loeb lend no support to this 

 idea ; in vertebrates as well as in crabs he believes that the galvano- 

 tropic phenomena are due to alterations produced by the current in 

 the action of associated groups of muscles, whereby movement of the 

 body or turning of the head towards one pole is rendered easier than 

 towards the other. 



