126 



COMPARATIVE ANATOMY. 



of the cud of the tail, and have a metameric arrangement like 

 that of the caudal muscles ; in the Mormyridae, for example, there 

 is on each side an upper and an under row of electric organs. 



The electric apparatus in all the above-named Fishes is to be 

 regarded from the same point of view both as concerns its mode 

 of development and anatomical relations: all electric organs are 

 to be looked upon as metamorphosed muscular tracts, 

 .and the nerve-endings belonging to them as homologues 



o o o o 



of the motor end-plates which are ordinarily found on 

 m uscles. 



As regards the minute structure of the electric organs, the same 

 essential arrangements are met with in all. The framework is 

 fornii'd of fibrous tissue, which, running partly longitudinally, 

 partly transversely through the organ, gives rise to numerous 

 polygonal or more or less rounded chambers or compartments. 

 These latter are arranged in rows, either along the longitudinal 

 axis of the body (Gymnotus, Malapterurus) or in a dorso-ventral 

 direction (Torpedo), forming definite prismatic columns (Fig. 102). 



FIG. 102. KLEeTiiic I'IMSMS UK '/'"//' "'" in'trnu i,-itt/i. (Semidiagrammatic. ) 



Numerous vessels and nerves ramify in the connective-tissue 

 lying between these compartments, the nerves being enclosed in 

 very thick sheaths, and having a great variety of origin accord- 

 ing to the species of Fish under consideration. In Torpedo, they 

 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 Gyinnotus, 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 of the latter, which are particularly well developed in 

 the last-named Fish. It is very remarkable that the electric 

 nerves of Malapterurus arise on each side from a single enormous 

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

 spinal nerve, is continued into a very lar^o primitive-fibre, which 

 pa-ses towards the, end of the tail, dividing as it goes. The 

 latter is invested by a thick sheath. 



On continuing our examination into the more minute his- 

 tological structure of the peripheral nerve-endings, it becomes 

 necessary to give a definition of those structures which are usually 



