420 Dynamic Theory. 



thetic nerves to the cells in the skin. These are two nerves running 

 longitudinally, close to the vertebral column, which connect with the 

 brain and spinal nerves, and ramify to all parts of the skin. If these 

 nerves are cut off at their roots, the chromatic function is destroyed, 

 although it may go on even with the spinal cord itself cut off just in the 

 rear of the brain. ( Semper. ) 



Professor Dewar has shown that the different colors of the spectrum 

 produce irritations in the retina, which are communicated to the optic 

 nerve as an electric current, and is denominated the "optic current." 

 ' < The intensity of this current, according to Dewar, is greatest under 

 yellow light, weakest under purple light, and nil in total darkness. " 

 Applying this to the chromatic action of the chromatophores, the an- 

 imal in darkness, or in dark surroundings, receiving no stimulus and 

 no resulting optic current, and consequently no contraction of the chro- 

 matophores, remains dark like his surroundings. Light surroundings, 

 on the contrary, will develop a current that will contract certain of the 

 chromatophores, or all of them, giving a corresponding lightness to the 

 color of the animal. If the light is reflected from colored objects, the 

 contraction of the brown or black chromatophores will be partial, and 

 the mixture of the color left in them, with the unchanged light-colored 

 chromatophores, will produce other and varying shades. 



Besides those named, the following possess the chromatic function : 

 Turbots and Flounders, Frogs, the Chameleon-Shrimp ( Mysis chame- 

 leon), Lizzards, Iguanas, Anolis, Cuttle fishes, Octopus, &c. , and 

 Chameleons. In many of these animals the function seems to be di- 

 rectly reflex, the action of light from the surrounding objects setting up 

 nervous action in a direct path from the eyes to the chromatophores, 

 without involving the general nervous centers ( brain, &c. ) in the stim- 

 ulation. But it is possible to excite the stimulation of the chromato- 

 phores in some of them, especially the chameleon, through the general 

 brain, by other stimulation than tha,t of light. Thus, if the animal is 

 at rest, his color simulates the objects with which he is in contact, but 

 if he be stirred up by some other excitement, as a defense or contest, 

 the stimulation overflows to the chromatophores, and he assumes all 

 colors, regardless of their protective quality. 



The chrysalis of a certain African butterfly ( Papilio Nireus ) has the 

 property of becoming colored to correspond more or less truly with 

 whatever body it may happen to be attached to. "A number of cater- 

 pillars were placed in a case with a glass cover, one side of the case be- 

 ing formed by a red brick wall, the other sides being of yellowish wood. 

 They were fed on orange leaves, and a bunch of the bottle-bush tree 

 was also placed in the case. When fully fed, some attached themselves 

 to the orange twigs, others to the bottle-bush branch, and all these 



