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



by Geoffroy St. Hilaire, M. Valenciennes, Ruclolphi and Peters, 

 proceed from the lateral nerve, a branch of the eighth pair. 



Thus there is nothing constant in the Origin of the nerves of 

 the electrical apparatus, as they proceed sometimes from the 

 eighth and ninth pair {Torpedo), sometimes from the eighth pair 

 alone {Silurus), sometimes from the pairs which arise from the 

 spinal marrow {Gi/mnotus and Rata). Their situation has also 

 no constancy, as they are sometimes situated towards the head 

 {Torpedo), around the body {Silurus), and on each side of the 

 tail {Gi/mnotus and Raia). 



The vessels of this organ are numerous and curiously arranged. 

 Between the articulation of each vertebra there passes a vessel, 

 alternately an artery and a vein, proceeding fi-om the principal 

 artery 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 ^^essels follow the course of the nerves, 

 and penetrate with them into the apparatus. Several branches 

 ramily 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 sui'face. 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\j 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 

 Gym7iotus. [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 Moi'- 

 mynis.'] 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. 



