354 ANATOMY OF VERTEBRATES. 



Gymnotus, the batteries are horizontal and the plates vertical, 

 fig. 233, A, whilst in the Torpedo the batteries are vertical and 

 their plates horizontal, fig. 231, E. The situation of the organs 

 is also very different in the two fishes ; they extend from before 

 the pectoral fins to the anterior part of the head in the one, 

 fig. 139, E, and from behind the pectoral fins to near the end of 

 the tail in the other, fig. 232, k. But a more important difference 

 exists in the condition of the interspaces between the delicate 

 transverse plates. In the Torpedo they simply contain a fluid. 

 In the Gymnotus two strata of pyramidal cells diverge from a 

 common basis traversing each interspace, and terminate freely, 

 the one towards one plate, the other towards the opposite plate, 

 and divide the fluid into a ( pre-cellular ' and ( post-cellular ' 

 portion. The cellular basis is ( positive,' the post-cellular fluid 

 and the partition-plate is negative, constituting the e voltaic 

 couple ; ' whilst the pre-cellular fluid is the conducting element 

 between one ( couple ' or plate and the next : the whole represents 

 the ternary type of the voltaic pile. The Torpedo's structure is 

 according to the binary type. Another remarkable difference 

 is in the source of the nervous supply. In the Gymnotus the 

 electric organs are supplied by the s rami ventrales' of all the 

 spinal nerves, about 200 pairs, that issue in the course of 

 their extent; some of the filaments ramify upon the horizontal 

 membranes from their cutaneous margins ; but the greater part of 

 the nerves come from the deeper-seated branches which descend 

 upon the median aponeurotic partition-wall, and spread upon the 

 septa of the organ from within outwards. Yet the nervus late- 

 ralis, which is derived from the same cerebral nerves as those 

 which, in the Torpedo, supply the electric batteries, and which is 

 formed by similar proportions of the trigeminal and vagus, ex- 

 tends the whole length of the electric organs in the Gymnotus 

 without rendering them a filament; it is situated nearer the 

 spine, and is of larger size than usual, but Hunter was not 

 able to trace any nerves going from it to join those of the 

 medulla spinalis, which run to the organ. 1 The quantity of 

 nervous matter supplied to the batteries of the Gymnotus is 

 less than in the Torpedo : but more substance enters into their 

 composition. 



The proportional size of the electric organs is also much 

 greater in the Gymnotus than in the Torpedo : indeed, the proper 

 body of the Gymnotus is, as it were, a mere appendage tacked on 



1 LXXXI. 



