ON’ THE, CUTANEOUS PIGMENTARY SYSTEM OF THE FROG ‘62 
glass; while the other, which was from the first the darker of the two, still 
retained its original coal-black appearance. Half an hour after this had been 
done, the contrast between them was much diminished, partly in consequence 
of the dark one having become slightly paler, but much more from the paler 
having grown darker. After another half hour they were of precisely the same 
colour, and when another similar period had elapsed, that which was the darker 
to begin with was distinctly the paler of the two, being much lighter than at 
first, though still considerably darker than medium. A hood was now placed 
upon this animal, and that upon the other was removed, and both were replaced 
in the same light as before. This procedure occupied about ten minutes, and 
within seven minutes of its completion the creature which had the head un- 
covered was already the paler of the two, having grown decidedly lighter in 
colour ; while that on which the cap had been last placed seemed somewhat 
darker; and after another hour, while the latter was still of much the same 
dark shade, the former, with the head exposed, was very much paler, being about 
midway between the medium and the palest possible tint. An experiment 
of the same kind was performed upon another pair of frogs with very similar 
results, the details of which it is not necessary to mention. I afterwards found 
that the presence of the hood tends to check diffusion, or even in some cases 
to give rise to concentration of the pigment, probably by making the animal 
struggle to throw it off ; so that in one instance a frog which was put in a per- 
fectly dark place, immediately after the cap had been put on, grew much paler 
in the course of two hours. This circumstance prevents the skin from becom- 
ing as dark on the application of the hood as it would do if the head could be 
covered without exciting the animal. This, however, only renders the facts 
above mentioned more striking, so that they afford of themselves sufficient 
proof that the direct action of light upon the integument is incapable of affecting 
the pigmentary functions ; and thus the conclusion before arrived at receives 
complete confirmation from these experiments. 
There is of course nothing new in the fact that other functions besides 
vision may be excited in a reflex manner through the optic nerves ; the con- 
traction of the pupil, and the sneezing experienced by many persons on coming 
suddenly into bright sunshine, being well-known examples of such pheno- 
mena. On the other hand, the view that the cutaneous nerves are sensitive 
to luminous impressions was destitute of any support from analogy.’ 
1 In the chameleon, a part exposed to the sun becomes dark, while the rest of the body remains 
unaffected. I have little doubt, however, that this is due to the calorific, not the luminous rays. That 
heat does produce such an effect was lately demonstrated to me by Professor Goodsir upon a living 
chameleon, which, when held in broad daylight before a dull-red fire for a short time, grew much darker 
on the side that was warmed, but retained elsewhere its former pale green colour. 
