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.
Thus by intervals, the lungs, like a perfect ventilatory machine, repeatedly, and without intermission, keep time with systolic and diastolic motions of the heart, in exchanging fresh and foul air for the support of this wonderful animal structure. This is the function of breathing respiration, namely inspiration, or the drawing in the fresh air; and expiration, the expelling the foul air.
The heart may be considered as the grand perpetual pumping engine, constructed on principles, so as to have the least friction possible.
This curious pump, the heart, sends the fluid of life, the blood, to all the parts, even the remotest in the whole machine. The arteries are the canals of the conveyance; they are strong elastic tubes, whose fibres are of spiral direction, and receive from the heart a power of contraction; which is continued from the heart in the moment of the systole, even to the remotest and most minute ramifications; by which the blood is pushed along to the minutest extremities. This is immediately succeeded with a diastolic reception of a fresh quantity of blood; so that on succession, the diastole and systole is perpetuated not only in the heart, but throughout all the arteries, in a friendly correspondence over the whole system: And this constitutes the pulse.
The veins, however, are somewhat of a different texture from the arteries; whilst the arteries transport the blood by an elastic and pulse-like contraction, the veins on the contrary, receive it from them at the extremities, in an easy and imperceptible gradation; for whereas the fibres in the arteries are in a spiral direction, elastic and strong, the coat of the veins are tender, and the fibres more longitudinal.
Hence, the wounds in the arteries and veins differ greatly; for whilst the arteries are for ever in a strong agitation, the veins perform their functions with less exercise; and therefore the wounds in arteries are always attended with danger, whilst the veins heal easy. From the same reason also, the arterial blood differs from the venal; for as the arterial blood is sent to all the Parts, as the grand nutriment, from whence the glands secrete their peculiar humours; the veins only transport it back again, to prepare it for a new rotation. Whence the blood in the arteries is of a high colour, and of a more alkalascent nature, than that in the veins.
Collateral with the minute ramifications of the veins and arteries, over the whole system, are the lymphatic vessels, which separate from the blood as it passes along, a transparent lymphatic humour, adapted for the different parts, which they are connected with.[[6]] These vessels are materially different, from either arteries and veins in their structure, and justly to be compared to a continuation of little cells; so constructed, as to transport the humours, without admitting a return.