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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_Of the Joint Connection of the Brain, Heart, Lungs, &c._
These are the principal actions in the grand movement of the human automaton.
The nerves are the subtile springs of the sensation, by which the whole becomes sensible of irritation, and agitated to its functions. Their ramifications are extended _ad infinitum_, and so curiously distributed, that though there is not a part in the whole system exempt from their spreading, yet they are distributed with such regularity, that they don’t interfere with each other. A regularity absolutely requisite to the order in the animal œconomy: For whereever the vibration of the nerves is obstructed, life or motion must in consequence cease.
The lungs are the ventilators in the system: they receive and let in the air; they sift it from all its gross particles, and extract the useful parts from this universal element of life; thus they give from every little vesicle, a due proportion of extracted air to the blood by the adjacent veins; and by the neighbouring arteries, again extract the foul air from the circulated blood, which by the mouth and nostrils is expelled again; at that instant a fresh quantity of air is drawn in again for a new supply.