It is impossible to observe these phenomena
attentively without being persuaded that the power
and influence of the heart, extend through the
arteries to the capillaries, and through these to
the veins.
The experiment is made in the easiest manner.
A ligature being applied, in the gentlest way, round
the limb of the frog, the circulation is immediately
impeded in the weh, so as to render the phenomena
which I have described quite apparent. The pulsatory,
and the retarded or arrested movements, of
the globules of blood, may be varied, in an infinite
manner, by increasing or diminishing the degree
of tightness of the ligature.
The conviction rests upon the mind that the
pulsatory power of the heart is capable of moving
the blood through the arteries, the capillary vessels,
and the veins, even in the extreme parts of the
system. 2
2. Evidence of the Muscular Action of the
Arteries.
The fact of a perfect circulation in acardiac
foetuses and acardiac animals, seems strongly to
indicate an important influence in the arteries. It
is difficult to conceive of this phenomenon, as accomplished
otherwise than by the agency of these
vessels. Still it may he contended that much in
this acardiac circulation is effected by the capillaries.
The circulation in fishes and the Crustacea, also
leads to similar conjectures relative to the powers
of the circulation of the blood. The former tribe
have a simple pulmonic heart only, which sends
the blood through the branchiæ, whence it is
returned through vessels which, without any fresh
accession of power, unite, form the aorta, distribute
the blood to the remotest parts of the system, and
issue in capillaries, and these in veins, by which it
is finally returned to the heart. In the latter tribe
of animals, there is, on the contrary, but a single
aortic heart. This organ propels the blood through
the systemic capillaries, through the venous system,
and finally through the branchial capillaries and
veins, until the circle is completed. Such at least
must be our view of these phenomena, unless we
discover that the power of the heart is aided by
that of the arteries, or of the capillary vessels.
To these arguments from cases of monstrosity
and from comparative anatomy, must be added