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.
If the Finger be applied to that part of the Ribs where the pulsation is felt in an erect position; and if, at the same time, the Diaphragm be contracted strongly, the beatings become immediately weaker, because the Heart is pulled downwards by the Diaphragm.
If one lies on the left side, the point of the Heart is felt beating nearer the Spine of the Back; if we turn on our Backs, it is scarce perceptible; and if we lie on the right side, it intirely vanishes.
These alterations of the Heart’s situation in the Breast, are more remarkable in some persons than in others; and in general I have found, by repeated tryals, that they were most considerable in those who were most subject to the Night-mare.
When the Body lies supine, the Heart necessarily falls on the Vertebræ of the Spine; and therefore, by its own gravity, must compress the left Auricle and Pulmonary Veins, which, at that time, lie directly under its basis; and, by that means, the course of the Blood through the Lungs will be stop’d. Thus the Blood will be collected in the Pulmonary Vessels, and the right, or rather superior Ventricle, not being able to discharge itself into the Pulmonary Artery, will be oppressed by the Blood returning from the Extremities; which, being gather’d in the vessels about the superior part of the Heart, will increase its gravity, and consequently augment the cause of the obstruction. In this manner the return of the Blood from the Head will be prevented, the tender dilatable vessels of the Brain will be over-distended, the nervous influence obstructed, and the vital motions, in a great measure, if not altogether, stopt. This I take to be a real fit of the Night-mare, and in this manner it appears to be produc’d.