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



tinous substance presents regularly undulated strise which it 

 would be impossible to take for fibres. 



At the point at which we are arrived, it is impossible not to 

 recognise a great analogy between the semitrausparent gelatinous 

 substance which essentially constitutes the discs of the peculiar 

 organ of the Hays, and that of the prisms of the apparatus of the 

 Torpedo, the rhomboidal meshes of the Si/urus eledricus, and 

 those interrupted oues between the transversal and vertical fibrous 

 laminae of the Gymnotus. 



Although there may be differences in form between the discs 

 of the organ of the tail of the Rays and those which constitute 

 the prisms of the electrical apparatus of the Torpedo, these dif- 

 ferences are certainly less considerable than those of the poi'tions 

 of gelatinous substance circiimscribed by the partitions and 

 areolae of the ajiparatus in the Silurus and Gymnotus, which 

 however produce similar effects to those of the Torpedo. 



The mode of arrangement of these discs is as regidar in the 

 Ray as in the Torpedo, and approximate much nearer to the 

 latter than to that of the same parts in the apparatus of the 

 Silurus and Gymnotus. 



The nerves of this apparatus originate in the portion of the 

 spinal marrow which is prolonged into the caudal vertebrae. I 

 have an object in view in remarking that this portion of the 

 spinal marrow must be composed of sensitive and motive nervous 

 fibres, for it corresponds to the portion called cauda equina in the 

 higher animals. 



The nervous roots which originate from this organ do not take 

 their rise together at the same level, but there springs alternately 

 an anterior and a posterior root. It is always from the anterior 

 one (beibre its anastomosis with the postei'ior) that the greatest 

 number of nerves which exist in the apparatus proceed ; lastly, 

 some issue from the ganglion and the lowest branch of the two 

 ■which proceed from it. These nerves are of the number of four 

 to seven for each nervous pair. They are, as is seen, very nume- 

 rous, and their diameter is from i to |- millimetre. These nerves 

 are finally distributed in the thickness of the partitions which 

 separate the lateral muscles from the tail, when they penetrate 

 into the organ, after being more or less subdivided. In the Raia 

 ruhus and R. batis the greatest number penetrate into the longi- 

 tudinal pile of the internal surface ; in the Raia clavata they pe- 

 netrate into some one of the partitions of that surface. In these 

 three sjiccies several branches wind round the superior and infe- 

 rior margins of the apparatus to penetrate into one of the parti- 

 tions of its subcutaneous ])ortion. lu the first two species these 

 superficial branches freely anastomose before penetrating. 



it results from these facts that a considerable number of nerves 



