242 COMPARATIVE ELECTRO-PHYSIOLOGY 



Fritsch estimates the total number of these plates in some of 

 the Torpedos to be over 1 50,000. 



From the point of view of their development, these 

 electrical organs in general constitute modified muscles, 

 containing nerve-endings. The electrical fish known as 

 Malepterurus of the Nile is an exception to this rule, inas- 

 much as morphological evidence goes to prove that in its 

 case it is glandular, rather than muscular, elements which 

 have been so modified. 



The peculiar characteristic of the discharge of electrical 

 organs in general, is that it takes place in a definite direction 

 at right angles to the plates. It was Pacini who tried to 

 establish the generalisation that the direction of the dis- 

 charge would be found to be dependent on the morphological 

 character of the organ. He found that as a general rule 

 the discharge takes place in a direction from that surface of 

 the disc which receives the nerve (henceforth to be referred 

 to as the anterior surface) to the opposite non-nervous, or 

 posterior, surface. Thus in the Torpedo, where the plates 

 are horizontal, and the anterior or nervous surface constitutes 

 the ventral aspect of the disc, the discharge is from the 

 ventral or anterior, to the dorsal or posterior surface. In 

 Gymnotus again, the plates or discs are vertical to the long 

 axis. The anterior or nervous surface is here towards the 

 tail-aspect, and the discharge is from tail to head. If these 

 cases had been all, Pacini's generalisation, as regards the 

 direction of discharge from the anterior nervous to the 

 posterior non -nervous would have been complete, and from 

 it some attempt might have been made to offer an explana- 

 tion of the phenomena. Unfortunately, however, this is not 

 so, since Malepterurus presents a hitherto inexplicable ex- 

 ception to the rule. In this fish, though the anterior or ' 

 nervous surface is towards the tail-aspect as in Gymnotus, 

 yet the discharge is in the opposite direction towards the 

 head : that is to say, from the posterior surface to the anterior. 

 The difficulties in the way of an explanation of the activity 

 of these electrical organs of certain fishes are thus seen to be 



