INFLUENCE OF LIGHT. 327 



etc., to the surroundings of the animal, which is indirectly 

 the result of sight, as the conseqnence of which they are 

 in many cases protected from harm. The change of color 

 is due to certain color- or pigment-cells, which are more or 

 less ramified, and which under certain stimuli contract. 

 The pigment differs in color in different individuals and 

 species and in different parts of the body, being yellow, 

 brown, black, sometimes even red or green, while tlie color 

 changes during contraction and expansion. It is on this 

 distribution and stratification, says Semper, and their 

 alternate expansion and contraction, that the pattern 

 which the frog's skin displays at any given moment 

 depends. The irritation which excites tlie action of the 

 chromatophores takes effect only through tlie eyes and 

 optic nerves, and not directly on the pigment-cells. 



The under side of jilaice and flounders, as is well 

 known, is white, but the upper side exhibits the chromatic 

 function in a high degree. Pouchet found among a great 

 number of plaice a single dark-colored fish, in which the 

 chromatophores must have been in a state of permanent 

 relaxation. On closer examination he found that it was 

 totally blind, and thus incapable of assuming the color of 

 the objects around it, the eye being unable to act as a 

 medium of communication between them and the chro- 

 matophores of the skin. 



By experiments in severing the connection of some of 

 the spinal nerves with the sympathetic nerves of the same 

 side Pouchet succeeded in limiting the chromatic func- 

 tion to those spots where the nerves remained in connec- 

 tion 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 colors reflected 

 from surrounding objects. It was thus proved that the 

 sympathetic nerve, and not the spinal cord, is the conduc- 

 tor of the optical stimulus which causes the movements of 

 the chromatophores. (Semper's Animal Life.) 



