ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 59 
The natural interpretation of these results appeared to be, that there 
exists a constant tendency to diffusion of the pigment in a limb so soon as it 
is liberated from the influence of the usually recognized nervous centres. It 
afterwards occurred to me, that if this were really true, diffusion of the pig- 
ment might, by proper management, be observed in an amputated limb before 
the supervention of the tendency to post mortem concentration: for I knew, 
from reasons to be mentioned hereafter, that this effect of death depended 
on the cessation of the flow of blood through the vessels, and, from what 
I had seen of arterial contractions in the frog’s web, and vermicular move- 
ments of the mammalian intestine from a similar cause, I felt sure that, if the 
blood were retained within the vessels, the arrest of the circulation could not 
be instantaneous in its effects upon the pigment, but that some minutes would 
probably be required to develop them ; during which time the diffusion result- 
ing from liberation of the pigment-cells from the influence of the ganglia in the 
trunk would proceed unchecked. Accordingly, on September 3, 1858, having 
tied a string tightly round the ankle of a pale frog, I immediately amputated 
above the ligature, and, avoiding the loss of time involved in tying out the toes, 
placed the foot at once on a plate of glass with a drop of water, two adjacent 
toes being kept apart by morsels of moistened lint. Within a minute and 
a half of the application of the string, the pigment in the web was observed 
to be in the angular condition, with short simple projecting processes, 1.e. 
approaching stellate, and two minutes later two contiguous cells were sketched 
in that state. About a minute after this it was evident that diffusion was 
taking place, and it continued to develop itself during the next ten minutes, 
at the end of which time the rays of the stellate pigment had shot out com- 
plicated offsets. Within the following five minutes, however, it was arrested 
by post mortem concentration, which gradually carried the pigment back 
to the angular state. This experiment, therefore, furnished confirmation of 
the view, that, in the ordinary circumstances of the animal, the influence of the 
central organs of the nervous system is required for the maintenance as well as 
the development of concentration of the pigment in the limbs. 
Supposing this to be established, it would follow that the accommodation 
of the tint of the skin to that of surrounding objects is certainly not the result 
of direct action of the rays of light upon the pigment-cells, but a reflex pheno- 
menon ; and it was an interesting question whether the afferent nerves concerned 
were the optic pair, or branches in the skin sensitive to luminous impressions. 
With a view to determining this point, I completely removed the eyes of a pale 
frog on September 13, 1858, at I p.m., and then placed it in a dark cupboard. 
During the first hour after the operation it became even paler than before, 
