350 ANATOMY OF VERTEBRATES. 



The lateral membranous canals, p 9 p, from the eustacnian outlet, 

 diverge to the orifices of corresponding lateral bony canals, which 

 ascend between the basioccipital and basisphcnoid, and communi- 

 cate each with the transverse subdivision, s, of the posterior or 

 occipital branch of the median eustachian canal: a small rhomboidal 

 sinus is formed at the point of union, from which a short canal, 

 t, is continued to the tympanic cavity. The common inferior 

 outlet, situated on a prominence, is partly closed by a valve, ??, 



reducing its area to a crescentic form. 1 



^j 



G8. Electric Organs of Fishes.- -Besides the modifications and 

 appendages of the peripheral extremities of the nerves consti- 

 tuting organs of special sense, and those of which the function is 

 still conjectural, there are nerves in fishes subject to more extra- 

 ordinary combinations, and forming instruments, unknown in the 

 higher vertebrate classes, having the property of accumulating 

 and concentrating the subtle mode of force applicable to the com- 

 munication of electric shocks. The faculty is limited to few 

 genera, the most remarkable being Torpedo and Gymnotus, the 

 species of which possess the electric organs in the highest state of 

 developement. In a minor degree the organs and power exist in 

 J\Ialapterurus electricus, Mai. Beninensis s Mormyrus longipinniSy 

 JMor. oxyrhynchus, Mor. dorsalis, Trichiurus electricus, Gymnarchus 

 niloticus, and Tetraodon electricus. 



In the Torpedo Galvanii the organs are two in number, are 

 large, flattened, reniform bodies, lodged on each side of the head 

 and gills, and encompassed by these and by the anterior borders 

 of the pectoral fins (fig. 139, E) : they consist of a mass of ver- 

 tical, for the most part hexagonal, prisms, the ends of which are 

 covered by the dorsal and ventral integuments. Beneath these 

 the organs are immediately coated by a thin glistening aponeu- 

 rosis, which sends down partitions forming the chambers of the 

 prismatic columns. Each column, when insulated in the recent 

 fish, seems like a mass of clear trembling jelly; but consists of a 

 series of delicate membranous plates inclosed by, or adherent by 

 their margins to, a proper capsule, and separated from each other 

 by a small quantity of a limpid albuminous fluid. Each flattened 

 cell thus formed is lined by an epithelium of nucleated cells : 

 the fibrous tissue of the plates and common capsule presents the 

 microscopic characters of elastic tissue ; between it and the epi- 

 thelium is a clear unorganised layer, the seat of the ultimate 

 ramifications of the vessels and nerves. The proper capsule 

 adheres to the aponeurotic partition-walls which support the 



1 CLXXII. p. 521, pi?, xl. xli. xlii. 



