24 M. Robin on a peculiar Organ found in the Rays. 



ai'tery and vein of the tail. These two vessels never pass together 

 to reach their foramina, and they never traverse the inferior spi- 

 nal apophyses, like the nerves, to issue from the spinal canal. 

 Beyond the vertebrae, the vessels follow the course of the nerves, 

 and penetrate with them into the apparatus. Several branches 

 ramify on its surface, surround it with their anastomoses, and 

 from the plexus which they form, some branches are detached, 

 destined either for the skin or the adjoining muscles. 



Those vessels which penetrate the thickness of the organ are 

 there subdivided infinitely in the partitions of connective tissue 

 which separate the discs from one another. 



From the plexus formed by the arterial and venous ramifica- 

 tions capillaries are given off, which are directed towards the pos- 

 terior face of the disc which is in front of them, and penetrate 

 into its substance. A capillary vessel never penetrates into the 

 anterior surface of a prism ; but we have stated that the nerves 

 ramify opposite to or against that surface. The capillaries which 

 penetrate the discs are very elegantly arranged in flexuous loops, 

 and are sometimes agglomerated in the form of tufts. These 

 loops and tufts are lodged and buried in the cavities by which 

 the disc is hollowed out ; these excavations exist only on the pos- 

 terior face of the discs, whilst the anterior face against which 

 the nerves are arranged is smooth. The capillaries which are 

 buried in the discs are from y^y to ~j of a millimetre in dia- 

 meter. 



To sum up the matter, there exists in the Rays a pretty volu- 

 minous organ, situated in the tail of that fish, as in that of the 

 Gymnotus. [From a letter which I have received from Prof. J . 

 Miiller, Riippell would appear to have described an organ ana- 

 logous to the former in the tail of the fishes of the genus Mor- 

 myrus.l This organ of the Rays receives fine but very numerous 

 nerves. It is formed of a gelatinous semitransparent and firm 

 matter, as in all the electrical organs known. This substance is, 

 as in all these fishes, divided into polygonal discs, regularly piled 

 together, against which nerves ramify that terminate by succes- 

 sive bifurcations and anastomoses supplied from their elementary 

 fibres. How can we help seeing in this an electrical apparatus ? 

 It is true that its position is not the same as in the Torpedo, but 

 in the Gymnotus and the Silurus the organ is also situated in the 

 tail or around the body. These conclusions are further confirmed 

 by the following facts : I have proved that this apparatus is 

 wanting in the tail of the Torpedo and the genera Mu^telus, 

 Scyllium, Squatina, Zygcena, Acanthias and Carcharias. 



The presence of this apparatus in the tail of the Rays explains 

 the immoderate proportional length of this organ, its flattened 

 form beneath, and the absence of the inferior lobe on the caudal 



