ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 65 
the first effect of removal from the influence of the nervous centres in the head 
and trunk is arterial relaxation and pigmentary diffusion, followed in a variable 
time by contraction of the vessels and concentration of the dark molecules, 
giving place again to relaxation and diffusion, after which succeed irregular 
alternations of contraction and dilatation in the one case, and of concentration 
and diffusion in the other. Here, though the vascular and pigmentary changes 
do not at all correspond with one another in point of time, yet there is an evident 
parallel between them; and, admitting that in each case the variations are 
the result of alternate action and inaction of the appropriate local nervous 
system, it is evident that concentration of the pigment corresponds to contrac- 
tion of the muscular fibres of the arteries ; these being both the results of nervous 
action, while diffusion of the pigment, like arterial relaxation, takes place when 
the nerves cease to operate. It will be remembered that a similar conclusion 
was derived from the study of the influence exerted upon the pigment-cells by 
the central ganglia. Hence it appears that the tendency to diffusion of the 
pigment-molecules is in constant operation in the cells, but kept in check by an 
antagonistic concentrating agency varying in obedience to nervous influence. 
It is an interesting circumstance, that two functions seemingly so totally 
distinct as muscular contraction and pigmentary concentration, should both be 
thrown into a state of activity in consequence of arrest of the circulation. It 
is to be remembered, however, that there is no evidence that either the in- 
voluntary muscular fibre or the pigmentary tissue is directly influenced by 
the cessation of the flow of blood, the effect being apparently produced through 
the medium of the local nervous system. This we know with certainty in the 
case of the post mortem movements of the intestines ; and we have seen reason 
to think it likely that the same is true regarding the contractions of the arteries 
after death, and the concentration of the pigment under similar circumstances. 
It is a curious question how the arrest of the circulation causes these actions 
of the local nerves. The idea suggested by the facts is that the tissues begin 
to suffer from the want of fresh supplies of the vital fluid, and resent the injury, 
as it were, by a struggle. 
Rich in results of general physiological interest as the study of the pig- 
mentary system of the frog has proved, it has yielded fruit of not less value 
in a pathological point of view. Indeed, what induced me to investigate the 
functions of concentration and diffusion, was the unexpected light thrown 
upon the nature of inflammation by the effects produced by irritants upon 
those processes. For information on this subject I beg to refer the reader to 
my paper “On the Early Stages of Inflammation ’.’ 
1 See p. 209 of this volume. 
LISTER I F 
