192 



COMPARATIVE ANATOMY 



lying between these compartments, the nerves being enclosed in 

 thick sheaths, and having a great variety of origin according to 

 the species of Fish under consideration. In Torpedo, in which 

 the electric organs probably arise in connection with the great 

 adductor muscle of the mandible and the constrictor of the gill- 

 arches, the nerves arise from the " electric lobe " of the medulla 

 oblongata, a single branch coming also from the trigeminal ; in all 

 " pseudo-electric " Fishes, as well as in Gymnotus, in which over 

 two hundred nerves pass to the electric organ, they arise from the 

 spinal cord, and most probably are in the closest relation with the 

 ventral cornua, which are particularly well developed in the last- 

 named Fish/ It is remarkable that the electric nerves of Malop- 

 terurus arise on either side from a single enormous lens-shaped 

 nerve-cell, which, situated in the neighbourhood of the second 

 spinal nerve, is continued into a very large 

 primitive-fibre, which passes towards the end of 

 the tail, dividing as it goes. The latter is invested, 

 by a thick sheath. 



It is stated that in all electric Fishes the side 

 of the electric plate on which the nerve branches 

 out is negative at the moment of discharge, while 

 the opposite side is positive. Thus the different 

 arrangement of the parts in Gymnotus and 

 Malopterurus renders it clear that the electric 

 shock must pass in different directions in these 

 Fishes: in Malopterurus it passes from the head 

 to the tail, and in Gymnotus in bhe contrary direction, while in 

 Torpedo the discharge passes from below upwards. 



Experiments have shown that all electric Fishes are proof 

 against the electric current, with the limitation that muscles and 

 nerves even the electric nerves themselves separated out from 

 the body are capable of being excited by the current. ' The last 

 and most important question with regard to the electric Fishes 

 naturally concerns the mechanism whereby the electric plates 

 become temporarily charged with electricity. The reply to this 

 question, although probably not so difficult a one as that relating 

 to the mechanism of muscular contraction, is still far from being 

 answered" (Du Bois-Reymond). The only thing that can be stated 

 with certainty is, that the electromotive force is under the control 

 of the will. 



i 



FIG. 144. ELEC- 

 TRIC PRISMS OF 

 Torpedo <"/- 

 morata. (Semi- 

 diagrammatic. ) 



