96 THE INFLUENCE OF INANI5[ATE SUKROUNDINGS. 



as distinct from its companions as from the bottom of the 

 aquarium. Closer investigation proved that the creature was 

 totally blind, and thus incapable of assuming the colour of the 

 objects around it, the eyes being unable to act as a medium of 

 communication between them and the chromatophores of the 

 skin. 



Up to this point Pouchet's researches piesent nothing really 

 new. But he proceeded to investigate the natural question ; How 

 and by what course is the impression received by the eye passed 

 on from the optic nerve to the chromatophores located in the 

 skin 1 Two modes of transmission are here possible : one by 

 means of the spinal cord and the pairs of nerves distributed 

 by it to certain sections of the muscles and skin — these are 

 known as the spinal nerves ; the other by two nerves running 

 longitudinally close to the vertebral column, the sympathetic 

 nerves, as they are called, and which are closely connected 

 with the spinal nerves and the brain. Pouchet detected and 

 proved that the connection was not severed, and the chromatic 

 function was not interfered with, if the spinal cord was completely 

 divided close behind the brain, thus cutting off the first means 

 of communication between the eye, the optic nerve, and the 

 chromatophores. On the other hand, the chromatophores lost 

 their power of contraction completely if the two sympathetic 

 nerves only were destroyed at the root. These, as before 

 explained, are connected with the very finest nerves of the skin 

 — which, it would seem, extend to the chromatophores — by 

 means of the spinal nerves which are given ofl^ from the spinal 

 cord on each side at regular intervals. By severing the connec- 

 tion of some of these with the sympathetic nerve of the same 

 side, Pouchet succeeded moreover in limiting the chromatic 

 function, to those spots where the nerves remained in com- 

 munication with the sympathetic ; and he was thus enabled to 

 produce at pleasure a zebra-like marking on one side of a fish, 

 while the other side retained its natural hues and their normal 

 variation according to the colours reflected from surrounding 

 objects. In this way it was indisputably proved that the 

 sympathetic nerve, and not the spinal cord, is the conductor of 

 the optical stimulus which causes the motions of the chromato- 



