58 COMPARATIVE PHYSIOLOGY 



therefore independent of any concomitant nervous disturb- 

 ance. In the electrical organ, the P.D. v/hich accompanies 

 muscular or glandular activity has been elaborated into a 

 weapon of defence or aggression. The E.M.F. of each cellular 

 element in a gland or muscle is of very small dimensions. In 

 a frog's sartorious, where the parallel arrangement of the 

 elemicnts eliminates any summation of potentials, the cur- 

 rents of action or injury are rarely greater than 0*05 volt. But 

 it has been shown that by arranging several frog's muscles 

 in series with the cut surface of one opposed to the uninjured 

 surface of the other, a summation of the potential difference 

 due to injury can be obtained up to a volt or more. 



Electric organs have been studied in three genera of fishes 

 — a Mediterranean ray, Torpedo, Malapterurus, a catfish 

 of North African rivers, and Gymnotus, the electric eel of 

 the tropical zone of S. America and Africa. They consist 

 essentially of disc-like cellular elements richly supplied with 

 nerve-endings on one surface and arranged in columns in a 

 manner reminiscent of Volta's pile. In Torpedo there is an 

 electrical organ on either side of the head, consisting of hori- 

 zontal rov/s of discs which represent functionally modified 

 muscle fibres. In Malapterurus the electrical organ is 

 developed from unicellular glands situated beneath the skin 

 in the middle region of the body. In Gymnotus it is located 

 in the tail. The columns are arranged transversely in 

 Torpedo and longitudinally in the other two genera. The 

 shock delivered by the electric organ of Torpedo is equivalent 

 to that obtained from about thirty Daniell cells. Bernstein 

 and Tschermack, who studied the variation of the P.D. with 

 cooling and heating, found it to be (within physiological 

 limits) directly proportional to the absolute temperature, as 

 would be expected from Nernst's formula for the E.M.F. of a 

 concentration cell. 



Secretion of Poisons.— Secretion of poisonous substances 

 which are undoubtedly means of attack or defence is met with 

 throughout the animal kingdom ; and their study raises a 

 number of points of general biological interest, notably the 

 questions of anaphylaxis and immunity. Many poisonous 



