Thus the Alimentary Tube is mov’d by the irritation of the food, rarefied air, &c.
And in like manner respiration is carried on, by the uneasiness that is felt in the Lungs at the end of every dilatation and contraction of the Thorax, which is owing to the resistance that the Blood meets with, both from the collapsing of the Lungs, and from the pressure of the rarefied air on the small Pulmonary Vessels, during their expansion: to which may be added, the elasticity of the Cartilages.
These several stimuli can only be perceiv’d by a sentient principle, which, in the human species, is call’d the Soul.
When the Soul is first united with the Body, and receives command over the organs of Motion, it seems to have been laid under a kind of necessity, by which it is compell’d to exert these organs in avoiding whatever is hurtful, and in chusing whatever is apparently beneficial, to the Body.
’Tis evident, from the laws of the Circulation, that when the Motion of the Blood through the Lungs is stop’d, for a short time, the right Ventricle of the Heart must be violently distended, and consequently severely stimulated. This strong irritation may bring the Ventricle into a vigorous contraction, which is all that is wanted to put the admirable machine again in motion; for, as soon as the right Ventricle discharges itself into the Pulmonary Artery, ’tis plain, from the laws of hydraulics, that the Blood must move in the Pulmonary Veins; and therefore the pressure on these vessels must be overcome. Thus the circulation of the Blood will be renew’d, and the vast distention of the vessels about the Heart, will rouse the attention of the Mind to change the uneasy position of the Body as soon as possible; which will alter the direction of the Heart’s center of gravity, and therefore take the pressure off the Pulmonary Veins and inferior Auricle, and by that means afford a free passage to the Blood through the Lungs. In this manner people may recover, without any external assistance.
’Tis highly probable that the Motion of the Blood is renew’d before any of the Voluntary Motions are recovered; for we never find that any of the Voluntary Motions remain after the Motion of the Heart ceases; and the surprising process of generation shews, that the first Motion observable in animal Bodies, is that of the Heart[14]. We have many instances, in Brutes, of the Heart’s Motion continuing long after the action of the Voluntary Muscles is quite destroy’d[15]. It is not improbable, that the human Heart would contract itself after Death, if the same experiments could, with any degree of humanity, be tried on it, that are made on the Hearts of Brutes: and the great Lord Bacon gives an instance of a criminal’s Heart, which he saw, after torn from the Body, leap up and down for several minutes[16].