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.
94. The heart is well known to beat after death, if death be not the result of a gradual decay. Sometimes, indeed, its muscular irritability is so active that the heart will beat for hours. E. Rousseau observed it beating in a woman twenty-seven hours after she had been guillotined.[123] Not only will it beat after death, but in many animals even after removal from the body: the heart of a young puppy, or kitten, will beat for three or four hours after its removal; that of a full-grown dog, or cat, not one hour; whereas the beating of that of a tortoise, or a frog, will, under proper precautions, be preserved for days--and even after it has stopped, it may be stimulated to fresh pulsations.