SECT. IV.
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.
[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.