442 ELECTRO-PHYSIOLOGY CHAP. 



fresher and more healthy the animal (Torpedo) the signs being 

 arched back and slight protuberance of the contours of the 

 organs the more certainly, according to Schonlein, will it react 

 to every discharge. 



Since the electrical nerves may be excited by the discharge 

 of the animal itself as well as by artificial currents, the compara- 

 tive immunity of the electrical fishes might depend principally upon 

 quantitative differences in the threshold of excitation for their 

 nerves as compared with the nerves of other animals, and many 

 facts seem at first sight to support this view. Boll made com- 

 parative observations on the nerves of a frog's leg and the first 

 spinal nerves of Torpedo, tetanising them with the sliding induc- 

 torium according to Eosenthal's method for determining the differ- 

 ence of excitability in nerve and muscle. Contraction always 

 occurred in the frog's muscle at a greater, often far greater, dis- 

 tance of coil than in the Torpedo muscles. Humboldt has a 

 corresponding observation. To his surprise he failed to elicit 

 twitches from the exposed muscles and muscle-nerves of Gym- 

 notus with a simple circuit (silver-zinc), although he succeeded in 

 doing so under the same circumstances in other animals. 

 Schonlein has recently made legitimate objections to Boll's ex- 

 periment, protesting against the application, to the doctrine of 

 immunity, of his own experiments on the apparently sluggish 

 excitability of electrical nerves (supra). In any case, 

 further comparative observations in this direction are much 

 wanted before any sufficient explanation of the apparently deep- 

 seated immunity of electrical fishes to electrical discharges of 

 any kind can be given. That such really does exist appears to 

 Biedermann evident from the reaction of the most powerful 

 electrical fishes (Gymnotus and Malapterurus), the shocks of 

 which may be fatal to other animals. Partial attempts at ex- 

 planation are not wanting. Pfluger suggested that the animals 

 might be thrown at the moment of the discharge of their own 

 nerves, from the central organ, into a state of depressed excita- 

 bility parallel \vith anelectrotonus, and thus be steeled against the 

 shock. But, apart from other reasons, it would then be difficult 

 to see why electrical fishes should be " steeled " against the dis- 

 charges of other individuals, as also against artificial electrical 

 currents. The results of other experiments of du Bois-Pteymond 

 and Boll, in proof of the same idea, have been equally negative. 



