much the gravity of the air may promote, and its
levity retard it.” 1
Haller and Hunter, and, indeed many other
physiologists, observed the effect of inspiration
and expiration upon the condition of the jugular
and other large veins, the brain, &c. But to Dr.
Barry is undoubtedly due the honor of having
prosecuted the subject experimentally.
Having given this rapid sketch of the opinions
of those physiologists who have chiefly occupied
themselves in the investigation of the powers which
move the blood, I shall now proceed to the detail
of my own observations. It will speedily he perceived
how much is to be discovered, by carefully
observing one individual arc, of the powers which
move the blood through the whole circle; and it
will be seen how invaluable the batrachian and
fish tribes are for this purpose.
1. Of the Extent of the Influence of the Heart
in the Circulation.
If the circulation in the web of the frog be
carefully examined under the most favourable circumstances,
it will be seen to he very rapid in the
1 On the Air and Epidemic Diseases. Preface, pp. ix—xii.
WHICH CIRCULATE THE BLOOD. 75
arteries, much less so in the capillary vessels and
in the veins ; and equable in all.
I have very seldom, however, if ever, seen the
circulation so perfect as not to be able to detect
with a good microscope, a degree of pulsatory
acceleration of the blood in the arteries at each
contraction of the heart. Judging from my own
observations entirely, indeed, I should have stated
that the natural circulation was rapid and slightly
pulsatory in the minute arteries, and slow but
equable in the capillary and venous systems.
If the circulation be, on the other hand, in the
slightest degree impeded, the pulsatory movement,
at each systole of the heart, becomes very manifest.
It is seen in all the three systems of vessels, arterial,
capillary, and venous. In the arteries there
is generally an alternate more and less rapid flow
of the globules, at each systole and diastole of
the heart; in the capillaries and veins, the blood
is often completely arrested during the diastole,
and again propelled by a pulsatory movement,
during the systole of that organ. In other cases
a pulsatory movement is seen in the arteries only,
the blood in the capillaries and veins moving
slowly but equably.