There takes place here the reverse of what happens in the general
capillaries; the fluid changes from black to red. We have already some
data upon the causes of this phenomenon; but I think new experiments
should be made before a thorough explanation can be given. This is so
much the more necessary, because if we knew how the black blood becomes
red, it would seem that we might know how the red becomes black.
I have stated the phenomena of this change of colour in my work upon
Life and Death; it would be superfluous to repeat them. There will be
found there also many details upon the circulation of the two capillary
systems, which I shall not mention here.