quite obvious upon the motion of the blood in the
capillary vessels, as I have already stated.
Not less is the various influence of the struggles
of the animal, in retarding, or arresting the flow
of blood in these vessels, or rendering it retrograde.
Any cause of contraction in the membranes
forming the web itself, has also a remarkable
influence upon the course of the blood in the
veins and capillaries.
If the limb be bound with a tight ligature, the
blood is rendered retrograde in its movement along
the minute veins and the capillaries, by slight
degrees of pressure made upon that part of the
limb which intervenes between the ligature and the
web. The blood is thus simply pressed out of the
large vein of the limb in a retrograde course
along its roots and the capillary vessels. Nothing
can be more convincing than this simple experiment,
that the circulation in the capillaries
is completely under the influence of slight external
impressions.
In attentively observing the circulation in the
web of the frog, we frequently remark anastomosing
branches between its veins. In these branches the
course of the globules of blood varies at every
moment, and in every possible manner : sometimes
this course is from one vein into the other;
sometimes it is retrograde from the second into
the first; sometimes it pursues two opposite
courses, from, various intermediate points, into each
of the two veins.
After observing this phenomenon attentively, we
are prepared for detecting other similar phenomena
in the capillaries themselves; and especially in
those occupying spaces between two adjacent veins.
Every kind of movement in the globules of blood
may be observed in these vessels, the obvious and
distinct result of forces impressed upon them, or of
issues afforded them.
Still more singular phenomena are produced by
arresting the course of the blood along a large
vein. Such a vessel immediately assumes the character
of an artery, apparently giving off branches,
instead of receiving roots—the globules of blood
pursuing a retrograde course.
In vain we seek for evidence of a power to contract
and propel the blood, in any of these phenomena
of the true capillary vessels. Where then is
such evidence to be found ?
Dr. Philip argues for a contractile power in