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. CHAP. III. _An account of the Symptoms._ Having now discover’d what appears, to me, to be the immediate cause of the Night-mare, viz. the pressure of the Heart on the left or inferior Auricle and Pulmonary Veins, which stops the motion of the Blood through the Lungs, and occasions a general stagnation; let us examine how that hypothesis will account for the several Phænomena or Symptoms, mention’d formerly in the description of this Disease. The first Symptoms that occur in that catalogue, are frightful Dreams, which generally are the forerunners of this Disorder. “In hoc genere (Somniorum) est Εφιαλτης quem publica persuasio quiescentes opinatur invadere, ac sentientes pondere suo gravare[12].” I shall not here undertake to solve that Phænomenon, which has so long puzzled the Metaphysicians, nor pretend to account for all kinds of dreams in a mechanical manner.