ON THE CUTANEOUS PIGMENTARY SYSTEM OF THE FROG 63 
for acting on the pigment-cells, which, as we know from experiments before 
mentioned, are capable of being influenced through the sciatic trunk. Hence 
it appears that post mortem concentration is the result of the cessation of the 
flow of the blood through the vessels, and that it is a purely local phenomenon 
developed in some manner quite independent of the central organs of the 
nervous system. 
The period at which it occurs varies a good deal in different cases. This 
seems to depend partly upon whether the blood is retained in the vessels or not. 
Thus in one instance in which a piece of web was cut out, so as to ensure com- 
plete escape of the vital fluid, the process was already considerably advanced 
within nine minutes ; whereas in the case above related, in which the blood was 
retained in the limb by a ligature, concentration did not commence till full 
a quarter of an hour after amputation. The season of the year also seems to 
have a great effect. In a cold room, in the depth of winter, I have known 
some hours elapse before the pigment began to change in an amputated limb : 
this is probably owing to greater languor in all the vital processes during the 
period in which the creature naturally hibernates. 
The dead frog, if previously healthy, assumes after a while a nearly uniform 
pale colour, concentration being carried to the extreme degree in all parts. 
It does not, however, remain in this condition; for when a variable time has 
passed, the skin becomes again somewhat darker, and on microscopic examination 
the pigment is found pretty uniformly angular or stellate. Nor are these the 
only changes to which the pigment is liable after death, as I first became aware 
in April 1858, when examining an amputated limb with reference to the post 
mortem contractions of the arteries, the blood being retained in the vessels. 
In that case, after complete concentration followed by slight diffusion had taken 
place, irregular changes began to appear ; some tracts of the web under observa- 
tion becoming affected with more or less full diffusion of the pigment, while 
in others it became more concentrated. Then after the lapse of some hours 
its state was found reversed, being concentrated in parts where it had been 
diffused, and vice versa. These curious variations continued till so late 
as the tenth day after amputation, becoming more frequent after the first 
few days; so that sometimes a considerable alteration was observable within 
half an hour. 
These facts appeared to me of great importance, as proving the con- 
tinuance of vital actions for a much longer time than had been previously 
supposed possible in a severed portion of the body. They seem also valuable 
with reference to the influence of the nervous system over the pigmentary 
functions ; for the circumstance that considerable patches of the web usually 
