The blood continues to flow out of these vessels for
a short time ; it then remains motionless in them,
unless impressed by some extraneous cause.
This experiment, carefully made, appears to
me almost decisive of the question of the absence
of irritability in the true capillaries. In order
that it may succeed perfectly, it is essential that
the animal should be vigorous, that the web
should not he too tightly extended, and that no
time should he lost in making the observations.
If the circulation be feeble, or the web at all too
tightly stretched, the circulation ceases entirely
in all the series of vessels.
Any struggles on the part of the animal must
be carefully observed, and their effects' distinguished
from those peculiar to the experiment.
The experiment of Haller and of Spallanzani
consisted, not in the ligature, hut in the division
of the larger vessels. In such an experiment, the
flow of blood is retrograde in the arteries, and
thus the phenomena are complicated. The action
of the heart is equally well excluded by a ligature,
and the result is of a more simple and satisfactory
character.
P. S. 2. In the second place I would remark,
that the due diffusion of the blood in the minute
and capillary vessels, appears to be regulated by a
principle of tension subsisting between the contents
of these vessels, their parietes, and the integuments.
It is on this principle that the blood leaves the
vessels of the web as the powers of life decline.
It is on this principle that the blood flows
into other channels when its proper channel is
obstructed.
It is on this principle that the blood flows
in all directions to the point at which a vessel
is wounded or opened, as in the experiments
of Haller and Spallanzani.
It is on this principle that there is an apparent
circulation in the minute vessels after the excision
of the heart, or the division of the large vessels
of a limb. This movement of the blood is
towards the point of division, and therefore
retrograde in the arteries.
The effects of increased tension in the integuments
are readily seen on extending the web
more or less tightly : a feeble circulation is
perfectly arrested, and the most powerful circulation
is greatly modified, by this means.