_Of the Circulation of the Blood._
We shall now consider the circulation of the blood; the grand source of
our existence. At the contemplation of which we must be lost in
admiration, at the wonderful wisdom of its contrivance; and with
uplifted eyes adore the cause of its motion!
The Heart, as I before observed, has two chambers or _ventricles_,
distinguished by the right and left; each of these chambers have also a
valvular cavity call’d _auricle_, or ear. Each _ventricle_ opens itself
into an _artery_ and _auricle_; and each _auricle_ opens itself into a
_ventricle_ and _vein_. These openings have valves, which open and shut
alternately, and by this, prevent any confusion or hindrance in the
influx and efflux of the blood.
The right _ventricle_ is the longest and greatest, but weakest: From
this ventricle springs forth the _pulmonary artery_, which spreads
itself throughout the Lungs.
The right _auricle_ is also the largest; in which opens itself the large
vein, called _vena cava_; as the common trunk that receives the blood
from all its branches spread over the whole human body.
The left _ventricle_ is shorter, but the walls are considerably stronger
than that of the right _ventricle_. Here begins the _aorta_, or the
great artery, by which the blood is sent to all the parts of the human
body.
The left _auricle_ is also somewhat smaller than the right; and in that
opens itself the _pulmonary vein_, which returns the blood from the
lungs, after it has received its necessary assistance from the vesicles
of air in the lungs.
This premised and understood, the circulation is performed in the
following manner:
The right _ventricle_ being full of blood, contracts itself. This
contraction is called the _systole_, by which it forces the blood
through the _pulmonary artery_ into every part of the lungs, even into
every vesicle; where it receives a portion of the air, drawn in by the
wind-pipe.
At the end of these _arteries_ join the ramifications of the _pulmonary
vein_, which receives the blood from the ramifications of the arteries,
by _anastomosation_; which, by little quantities, these veins gather
gradually into little branches, and at last become one common trunk,
which empties the blood into the _left auricle_ of the heart.
From the _left auricle_ it is let, by means of a peculiar constructed
_valve_, into the _left ventricle_; which, by a dilation called the
_diastole_, receives it from its _auricle_.
Immediately upon this reception, the _systole_ or contraction takes
place, and the blood is forced into the great universal artery, _aorta_;
by which it is farther transported into every part of the human body.
This _artery_ spreads as it goes, and continually decreases in
magnitude, as it becomes numerous in branches; until it terminates into
the smallest ramification in the remotest part of the body.
The _systolic contraction_, and the _diastolic dilation_, which
alternately take place in the heart, continue throughout the arteries
from the great trunk to the remotest capillary ramifications,[5] by
which means the blood is transported with more certainty and facility to
the minutest part.
Footnote 5:
Concerning this great act of continued systole and diastole,
Anatomists have greatly varied; but from the structure of the
arteries, and the continued pulsation, this way of transporting the
blood is confirmed both by reason and experience.
At the extremity of these arteries the veins take their commencement, in
ramifications equally small with the former; these take up the blood
from the arteries, by many infinite small quantities, and carry it
gradually back from those extremities to the heart again, for a new
rotation.
These veins commencing infinitely small, but gradually gathering, become
branches, and at length form one general trunk, called the _vena cava_,
(or the great universal vein); and this empties itself again into the
_right auricle_; from whence it is let again into the _right ventricle_,
whence it came; then again forced into the lungs; thence back again into
the left ventricle; from thence all over the whole body, and continues
the Circulation.
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