194 NATURAL SCIENCE. „^^. 



of colour in the upper side of flat-fishes and the absence of chromato- 

 phores from the lower side. He might conceivably have been able 

 to prove that there was a connection between these two phenomena, 

 but he has not attempted to do this. He refers to Pouchet's paper 

 on the Changements de Coloration sous VInfluence des Nerfs ^ as 

 indicating such a connection. He states that Pouchet studied 

 chromatophores especially in relation with the " atrophism of the 

 colour on the blind sides of Flounders, pointing most plainly to the 

 partial atrophy of the great sympathetic nerve, effected during the 

 passage of the eye from the right to the left, or vice versa, as the cause." 



It will, perhaps, seem incredible, though the truth of the state- 

 ment can easily be verified, that Pouchet, in the paper to which I 

 have referred, says not a word concerning the cause of the absence 

 of colour from the lower side of the Pleuronectid. It is strange, 

 indeed, that the distinguished son of the great author of the Poissons 

 fossiles, whose native language was French, should have fallen into 

 such an error in quoting a French memoir. Agassiz appears to have 

 forgotten for the moment that the optic nerves in a fish cross one 

 another at their origins from the brain, so that the eye of the right 

 side is connected with the left half of the nervous system and vice 

 versa. Pouchet particularly describes two experiments upon turbots 

 made expressly to ascertain whether one eye had more influence than 

 the other on the chromatophores of the upper or left side. It is 

 well known that Pouchet found the power in a fish of accommodating 

 the general tint, i.e., the lightness or darkness of its skin, to that of 

 the ground on which it was placed, to depend upon the eyes. When 

 both eyes were removed this power of accommodation was permanently 

 destroyed ; the chromatophores were unable afterwards to contract 

 or expand. He found that the removal of one eye in Gobius niger did 

 not affect this power of accommodation — the chromatic function — in 

 the least. He removed the right or upper eye of one turbot, the left 

 or lower eye of another, and found that in both cases the chromatic 

 function was uninjured ; the upper side continued to grow light or 

 dark according as it was placed on light sand or dark mud. 



Pouchet, however, further demonstrated by experiment that the 

 connection between the eyes and the dermal chromatophores was 

 effected exclusively by the great sympathetic nerve cord or chain of 

 ganglia. If the sympathetic plexus in the inferior vertebral canal of 

 a turbot was divided by cautery, the chromatophores of the skin 

 behind the section were paralysed. Agassiz may have supposed that 

 although both eyes were connected with each sympathetic, yet the 

 migration of the lower eye in someway affected the sympathetic of the 

 lower side, and that each sympathetic was connected with the chromato- 

 phores only of its own side. But Pouchet says nothing about all this, 

 and Agassiz makes no mention of any evidence that the alteration 



^ Archives de Physiologic et d' Anatomic, 1876. 



