62 ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 
From the part taken by the second pair of nerves in bringing about the 
changes in the tint of the skin under the influence of light, and also from the 
darkening of the hind legs observed to occur after dividing within the canal the 
roots of the branches which supply them,' we learn that the cerebro-spinal axis 
is chiefly, if not exclusively, concerned in regulating the functions of the pig- 
ment-cells. Considering that those functions have probably a close affinity 
with the processes of secretion and nutrition, it is interesting to find that they 
are thus subject to the control of the spinal system. 
The circumstance before alluded to, that a dark frog always becomes pale 
after death, is mentioned both by von Wittich and Harless, but without any 
discussion of its cause. This post mortem concentration takes place in a limb 
in spite of amputation, and therefore cannot be due to the agency of any ganglia 
contained in the head or trunk. Neither can it be the result of failure in action 
on the part of such ganglia; for if the circulation be artificially arrested in 
a part of a living frog without interfering with the nerves leading to it, a similar 
change in the pigment to that which results from death comes on before the 
nerves have become, so far as we can judge, at all impaired in their functions. 
This was proved by the following experiment :—On June 7, 1858, having tied 
the right femoral artery of a moderately dark frog in the middle of its course, 
I divided it below the ligature, and also cut through, in the same situation, all 
the soft parts of the thigh except the sciatic nerve with a little adherent muscle. 
The operation was completed at noon, when the animal was put into a dark 
place; and at 1! 4om p.m. the body generally was darker, but the right leg 
from the wound downwards was decidedly paler than before; the animal, 
however, still moved it freely. At 64 20m p.m. the general surface was as dark 
as ever, but the right foot presented the extreme degree of pallor; yet the 
creature still moved the leg both spontaneously and when the toes were 
pinched, showing that the motor and sensor nerves retained their functions. 
Sensation, however, was not so acute as in the left foot ; in the latter a touch 
sufficed to induce movements in the body generally, whereas in the former 
a pinch was necessary to produce the same effect. At Io! 15™ p.m. the same 
contrast in colour continued, but no movement could be induced in any manner 
in the pale limb, although obscure indications of a certain amount of sensibility 
remaining in it were still elicited by forcible pinching. 
In this case, concentration of the pigment came on in the limb in con- 
sequence of arrest of the circulation through it, several hours before its nerves 
concerned in sensation and motion had lost their powers, and therefore at 
a time when we cannot doubt that the ganglia in the trunk had full opportunity 
eee Puy, 
