xi ELECTRICAL FISHES 421 



above. Here, as in muscle, galvanometric evidence alone is 

 reliable. James Stark (cf. 32) was led to the discovery of the 

 electrical organ of the ray on hearing a fisherman say that a 

 shock resulted from touching the tail of the living animal. It is, 

 in fact, easy with the galvanometer to determine the fairly 

 energetic action of the organ. If a living ray is stretched, with 

 its ventral side downwards, upon a board (shaped like a draw- 

 net), the body being then immersed in sea- water, so that only the 

 tail projects beyond the handles of the board, it is easy to apply 

 two unpolarisable electrodes corresponding with the ends of the 

 organ. During rest there is, as a rule, little or no difference of 

 potential. Mechanical stimulation of the skin, on the other hand, 

 always produces a discharge of such intensity that even a small 

 fraction (YOTF) f the current is sufficient to drive the scale out of 

 the field (Burdon-Sanderson and Gotch, 13 c). In the leading-off 

 circuit the current passes from posterior to anterior electrode in 

 the organ itself, therefore it is antero- posterior. In the Mor- 

 myridcv, as pointed out by Fritsch (12 i), the electrical current 

 flows in the body from tail to head, i.e. in the same direction as 

 in Torpedo and (rymnotus. Specimens 15 and 20 cm. long 

 produce, as Babuchin remarks, " hardly perceptible twitches in 

 the rheoscopic frog's leg, while fish of 40 and 50 cm. evoke 

 sharp, maximal twitches, and can be felt by man, although 

 not more distinct than from a torpedo 10 cm. in length." In 

 vigorous and selected animals Fritsch was able to detect dis- 

 charges with the frog-alarum, when the electrodes, dipping into 

 the water of the holder, were brought as near as 20-30 cm. 

 without actually touching it. 



A. v. Humboldt had already pointed to the possibility of 

 a partial discharge of the electrical organ : he noted that 

 only one of two metal rods at 1012 mm. distance from the 

 gymnotus received the shock ; the other not. C. Sachs placed 

 four toads' legs on four different points of a gymnotus taken 

 from the water. All four twitched with strong discharges, but 

 if weak shocks were provoked by picking at the skin of the 

 tail, the hindmost preparation alone contracted. In view of the 

 innervation of the organ of Gi/mnotus, its " local discharges " 

 (du Bois-Eeymond) are easily interpreted, while it seems equally 

 clear that the Mcdapterurus organ can only function as a whole. 

 C. Sachs found a striking difference in regard to the strength 



