v.] THE ORGAN-DISCHARGE 79 



it actually is a battery of which the elements (discs) are dis- 

 posed in series. Further, the direction of discharge, excepting 

 in the somewhat doubtful case of Malapterurus, is always such 

 that current passes (in the animal) from the nervous to the 

 vascular surfaces of the discs. Thus, in Torpedo, where the 

 nervous surface is ventral and the vascular surface dorsal, 

 the discharge is from venter to dorsum. In Gymnotus the 

 nervous surface is posterior, and the discharge is from tail to 

 head. In Raia (the skate, which possesses a well-formed, if 

 attenuated, pair of electrical organs) the nervous surface is 

 anterior, and the discharge is from head to tail. This relation 

 between position of nervous plate and direction of discharge 

 is called after its discoverer " Pacini's law." The discharge is 

 attributable to a sudden action of the nervous surfaces, which, 

 while active, play the part of a series of zinc plates in a 

 voltaic pile. The electro-motive force in the discharge of a 

 single disc has been estimated to be 0.03 to 0.05 volt,* and 

 it is by reason of their columnar arrangement and large 

 number, that the high electro-motive values of the organ- 

 discharge are reached 200 volts from a column of 500 plates 

 is an E.M.F. estimated by Gotch and Burch in the case of 

 Malapterurus. 



The discharge can be brought about experimentally by 

 indirect (reflex) and by direct excitation. If, as was first done 

 by du Bois-Reymond, a longitudinal strip of organ be excised, 

 placed between unpolarisable electrodes, and directly excited, 

 it will respond to both directions of excitation by a discharge in 

 one given direction, that namely of the normal discharge. This 

 photographic record (Fig. 36) gives the responses (both in head 

 to tail direction) to induction shocks in positive and negative 

 directions, in the case of a portion of the rudimentary (or 

 vestigial) organ of the skate. The similarity between these 

 effects and the effects above described in the case of the eyeball 

 and the skin, are sufficiently obvious, and you will readily under- 

 stand how it has come about that I have compared the " blaze- 

 currents " of the eyeball, and of the skin, and of other organs 



* Gotch, ScJiafet J s Text-Book of Pliysiology, vol ii., p. 584. 



