Tho’ the contraction of the Heart is evidently the efficient cause of the Blood’s motion, and consequently of the secretion of these spirits in the Cerebellum, yet, without these spirits, the action of the Heart could not be performed. These two causes appear to act in a circle, and mutually depend on each other. Hence Hippocrates divin’d, ὁλον το ζωμα κυκλος εστι. These also convey the idea of a perpetuum mobile; since, as long as life lasts, an animal is really such, and far excels any machine that human art has been yet able to make, or (in the opinion of many philosophers) will ever invent.
The laborious Hoffman ascribes a great deal to this pressure on the Brain, where he says, “[9]Declivior cubitus sanguinis regressum quodammodo impedit, quia per venas jugulares descendere debet, quod elatiori capite commodius peragitur. Hinc, capite nimis demisso ac depresso, profundiores somnii cum insomniis, fiunt, universo corpore torpor inducitur. Eadem ratione, si quis facie prona velut in mensa, in somnum delabitur. Ob difficiliorem sanguinis regressum, gravitatem capiti, et ingenio stupiditatem accersit.”
“[10]Sed etiam mechanicæ causæ somnum producunt, compressio nempe Duræ Matris, aut Cerebri, quæcunque nata a Sanguine effuso, inpacto Osse, aquæ in Ventriculis copia.”
These, I hope, are sufficient to shew how far the motion of the fluids may be affected by the horizontal position of the Body; which, if duly consider’d, might be of great service in the practice of Physic; and perhaps many effectual derivations might be made, without drawing a drop of Blood. I saw a remarkable instance of this kind in a gentleman of a full habit, who, being ill of a Fever, talk’d rationally and rav’d alternately, as his head was elevated or depress’d. In acute Diseases, when the motion of the Blood is very rapid through the whole Body, the Brain must suffer greatly, on account of the horizontal position, to which people in such cases are confin’d; because, the Blood rushing violently into the Arteries of the Brain, and its return being retarded by the Jugular Veins, will remarkably contribute to produce delirious symptoms, so frequent in acute Disorders, which might be in some measure prevented, by raising the Head; for, by that means, the motion of the Blood through the Jugular Veins will be increas’d, the pressure on the Brain will be eas’d, and a safe and sudden derivation from the Head may be made, which may produce very happy effects, where no evacuation could be safely attempted.
Let us next take a view of the Heart, and consider how it may be affected by the various positions of the Body, particularly the supine one, in which the Night-mare generally invades.
The Heart is placed above the Diaphragm: the greater part of it lies in the left cavity of the Breast: its apex or point is turn’d towards the extremity of the sixth true Rib, where its pulsations are commonly felt: it adheres to the Lungs by its large vessels, and is connected to the Diaphragm by the Pericardium[11].
Thus the Heart is suspended in the Breast; and therefore must be subject to the laws of pendulous bodies, which alter their situation according to the different directions of their centers of gravity.
From the above just description of the human Heart, ’tis evident, that when the Body is erect, the parts of the Heart which are commonly called the right and left, ought to be more properly call’d the anterior and posterior.
Hence, when the Body is plac’d on the Back, these become the superior and inferior parts of the Heart.
That the Heart alters its situation in the Breast according to the different positions of the Body, and the different directions of its center of gravity, may be prov’d by the following easy experiments.