288 THE BODY AT WORK 



ELECTRIC ORGANS. 



Muscle disperses energy in the forms of mechanical work, 

 heat and electricity. Its structure, as already pointed oat, is 

 peculiarly favourable for the display of electromotive force. 

 In certain fishes muscle is so modified as to give an electric 

 discharge without developing mechanical work. The pro- 

 duction of an electric change is a by-phenomenon of muscular 

 activity. It becomes the sole function of an electric organ. 

 If the skin be removed from the tail of a skate, a cylindrical 

 column of brawny tissue about the size of a finger will be found 

 embedded amongst the muscles near its root on either side. 

 These are electric organs, although so weak that it is barely 

 possible to feel the shock which they give in a live fish. The 

 nearly allied Torpedo of the Mediterranean has far more power- 

 ful batteries. They are situate near its gills, occupying the whole 

 thickness of the fish from skin to skin. When the back of a 

 torpedo is pressed, it discharges a current of 30 volts, or even 

 more. Still more violent are the shocks given by an eel 

 Gymnotus which haunts the tributaries of the Amazon, a 

 terror to all who have to cross their fords on foot ; or the 

 African fish, Malapterurus. The current which these animals 

 develop attains an intensity of 200 volts. With the excep- 

 tion of those of Malapterurus, all electric organs are modified 

 muscle, and closely similar in structure. The organs of 

 Malapterurus appear to be modified glands. The skate's 

 electric organ may be taken as typical of the rest. When 

 sliced with a knife, it is seen to be divided by firm connective 

 tissue into minute chambers. These chambers are piled into 

 hexagonal columns, which lie lengthwise in the organ (they 

 are set dorso-ventrally in Torpedo). Each chamber contains a 

 jelly-like substance which embeds an electric disc. The disc 

 divides the chamber into a smaller anterior and a larger 

 posterior compartment. Each chamber is supplied with 

 several nerves which ramify into innumerable twigs on the 

 front surface of the disc. The development of the disc must 

 be considered for a moment if its structure is to be understood. 

 It starts life looking as if it would grow into a voluntary 

 muscle-fibre. A nerve joins it, forming an end-organ in the 

 usual way. Then the end- organ increases its spread unduly, 



