The next phenomenon in the modified circulation
is oscillation. This may be either slight, or
quite extraordinary.
A slight degree of oscillation frequently follows
the state of mere pulsation; or is observed as the
powers of the circulation fail. The heart seems to
propel the blood into the remote vessels, during
its systole ; during the diastole the motion of the
blood is probably rendered retrograde by the elasticity
and pressure of the parietes of the vessels or
of the textures of the contiguous parts.
But the most extraordinary example of oscillatory
motion in the globules of blood, is seen in the
minute arteries, on the subtraction of the action
of the heart, as in its ligature or excision. The
globules flow along the minute artery quite continuously
for many seconds,—probably during the
contraction or systole of its successive portions ;
it is then suddenly and rapidly moved in a contrary
direction in the same vessel,—probably by its
diastole.
The struggles of the animal produce extraordinary
effects upon the motion of the blood in
the minute and capillary vessels. The globules
in the capillaries are frequently quite stagnant;
the arteries frequently disappear suddenly; the
blood in the veins moves slowly, irregularly, and
frequently in a retrograde manner.
If a ligature be tied tightly round the limb, the
circulation in the web is arrested. By the gentlest
degrees of pressure on the limb, below the ligature,
the blood may be made to stagnate, to retrograde,
or to oscillate, in every possible way.
Similar appearances are produced, if the web
of the frog be stretched, or allowed to become
dry. The circulation is first enfeebled, and perhaps
made pulsatory; then it becomes oscillatory
and stagnant; lastly, some of the vessels disappear,
whilst others retain globules of stagnant blood.
In the former ease the vessels are probably simply
pressed upon by the drying web; in the latter,
they suffer some change in their parietes, not
sufficiently ascertained, but the object of a future
investigation. The web frequently becomes altogether
transparent and colourless.
The effect of loss of blood is very singular.
To the naked eye the web appears pale. Under
the microscope an Occasional globule of blood is
seen passing across the field along a semi-transparent
channel.
H