60 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
no doubt in consequence of the injury which had been inflicted, assuming 
apparently the lightest possible shade; and this continued with very little 
change till night, although the animal was still kept in the dark. Next morn- 
ing it was decidedly darker, and the tint was still deeper at 24 25" p.m. The 
glass containing the frog was now placed in a bright light, and surrounded on 
all sides by white objects ; but this change produced no difference in the colour 
of the skin, which continued till 7 30™ p.m. of a peculiar dingy hue. It was 
then put back into the dark place, and at 11" 4o™ p.m. was still exactly the 
same. On the following day, at 8a.m., the animal seemed a little paler, and 
was even lighter at 10 a.m., though still in the dark; so that it was evident 
that no difference whatever was produced upon its colour by admission or . 
exclusion of light. But that the nervous system generally was in a state quite 
disposed for acting upon the pigment-cells when subjected to appropriate 
irritation, was shown by the following circumstances. At the hour last men- 
tioned, the animal, having escaped from the vessel in which it was contained, 
struggled violently during my attempts to secure it, and in the short time thus 
occupied changed to almost the palest possible tint. It was then placed at once 
in the bright light, as before, but, in spite of this, was within ten minutes 
already decidedly darker, and, half an hour later, was almost coal black, though 
still subject to the full influence of white light. Just after this observation 
was made, the frog again escaped, and having again struggled considerably 
before it was replaced in the glass, it was seen to be within four minutes as 
pale as when first observed in the morning, but after the lapse of another 
half hour it was again almost as dark as ever, and continued so till 
2h 30" p.m., though all the while exposed to the same light. The observa- 
tions were continued for two more days, during which period the same 
complete indifference to the brightness or obscurity of surrounding objects 
was still evinced. 
These facts indicated pretty clearly that the eyes are the only channels 
through which the rays of light gain access to the nervous system so as to induce 
changes of colour in the skin. But for the sake of confirmation I thought it 
worth while to perform the following experiment. Two very dark frogs having 
been obtained, I put a hood of black cloth on the head of one of them, leaving 
the body and limbs uncovered, an aperture being made in the cloth below the 
throat for the purposes of respiration, and then placed them both in the same 
glass vessel exposed to white light. The struggles of the animal while the 
covering was being adapted and secured had the effect of making it grow 
much paler, so that it was of about medium tint when introduced into the 
* Probably from the irritation of the optic nerves. 
