Dr. Philip’s second argument for a contractile
power in the capillaries, is derived from observing
the effect of the application of various irritating
substances to the web upon the capillary circulation
and vessels. But nothing can be more
unsatisfactory than such an experiment. 1. The
pain inflicted, accelerates the circulation ; 2. the
struggles of the animal variously retard or
arrest it ; 3. the action of the irritant upon the
membranes of the web, upon the parietes of the
vessels, and upon the contained blood, is various
and complicated. It is impossible to determine
whether the action of the capillaries themselves
be augmented or diminished, or whether these
vessels, admitting them to possess any vital contractile
powers, be excited or debilitated, in so
complicated an experiment.
It is, therefore, obvious that whilst every fact
relative to the capillary circulation leads us to
ascribe its various phenomena to forces impressed
upon it from without, there is nothing
of satisfactory evidence or argument for an automatic
power in them.
WHICH CIRCULATE THE BLOOD. 91
5. Of the Influence of the Acts of Inspiration
and Expiration upon the Venous Circulation.
I have already quoted the singularly interesting
passage of Huxham, respecting the influence
of atmospheric pressure during inspiration, upon
the circulation or flow of blood along the
veins.
Dr. Barry justly observes, “ A vague unauthenticated
notion, that the return of the black
blood to the heart is, in some undefined way,
influenced by suction, may be traced as far back
as the time of Harvey. Haller, and many others
also, noticed a marked coincidence between the
inspiratory movements of the thorax in the warmblooded
mammalia, and the motion of their
venous blood. But the mechanism was never
pointed out, by which nature, in these animals,
applies the mighty agency of atmospheric pressure
to the veins, and connects, as cause and
effect, the expansion of the chest with the afflux
of the centripetal fluids to the heart. The experiments,
therefore, that demonstrate this mechanism,
and supply these important desiderata in
physiology, must be entitled to the meed of