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



of gelatinous substance circumscribed by the partitions and 

 areolse of the apparatus in the Silurus and Gymnotus, which 

 however produce similar effects to those of the Torpedo. 



The mode of arrangement of these discs is as regular 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 (before its anastomosis with the posterior) 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 pl*oceed 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 j 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 

 rubus and R, hatis the greatest number penetrate into the longi- 

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

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

 three species several branches wind round the superior and infe- 

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

 tions of its subcutaneous portion. In the first two species these 

 superficial branches freely anastomose before penetrating. 



It results from these facts that a considerable number of nerves 

 extend into the partitions of each series of discs infinitely subdi- 

 viding. From these subdivisions part the filaments which pene- 

 trate between the partition which separates each disc from that 

 with which it is in contact. This filament expands opposite to 

 the anterior face of each disc, but never does a single one pene- 

 trate into the substance of the disc. The nerves spread out on 

 the internal surface of the partition between it and the disc. No 

 single thread ever ramifies against the posterior face of the disc ; 

 we shall soon see that this surface receives only vessels. 



The elementary fibres of the nervous filaments have a double 

 character ; that is, they are true elementary nervous tubes tra- 

 versed by a semifluid substance which escapes in drops of variable 

 forms from their extremities when torn across. [These observa- 



