92. That nerves are stimulated by internal changes has long been recognized with reference to “subjective sensations.” The divided nerve, in that portion which remains connected with the centre, will at times cause great pain. Obscure organic conditions, changes of temperature, states of the blood, excite the nerves, and the patient feels as if the surface of the amputated limb were irritated. It is all very well to call these “subjective sensations”; that does not alter the fact of the nerve being called into activity by other than the normal stimuli from the surface; in like manner muscular movements (which are not to be explained as “subjective movements”) will be excited by organic stimuli when motor-nerves are separated from their centres. In each case it has sufficed that the nerve should be excited; and when excited, no matter by what means, the effect is always similar. 93. Here are a few facts. Stimulation of the nerves which send filaments to the chromatophores of the skin in reptiles causes the skin to become paler, and even colorless: the color-specks disappear under this contractile stimulus. This being known, Goltz deprived a frog of brain, spinal cord, and heart, thus eliminating all possible influence from them, slit up the skin of the back, and displayed the nerves which pass from each side of the spine to the skin; these nerves he then divided on the right side, and observed the skin on this side slowly become paler and paler, till finally it was as yellow as wax; the left side, having its nerves intact, retained its color. Two conclusions seemed to him warranted by this experiment: First, that even in the dead frog the nerves separated from their centre were still active; secondly, that the irritation of the nerves resulting from their section was the cause of the color-specks disappearing. This second conclusion was strengthened when he found that the irritation was increased when he cut the nerves bit by bit. It is not at present, I believe, clearly made out that the color-specks of the Cephalopoda are in direct connection with nerves; but it is tolerably certain that they are in some way under the influence of nervous stimulation, directly or indirectly. D’Orbigny, indeed, goes so far as to say they are dependent on the will of the animal.[121] This seems very lax language; but restricting ourselves to the fact of nervous influence, the experiments of Goltz receive further illustration in an observation I have elsewhere recorded.[122] I found that a strip of skin taken from the dead body of a calamary (_Loligo_) showed the color-specks expanding and contracting with vigor.