Heavy sighs and groans are the emphatic expressions of nature in distress, and generally arise from some obstruction in the Lungs; but in a fit of the Night-mare there appears a great accumulation of Blood in the vessels of that part, whence these Symptoms are easily accounted for. It may be observ’d of sighing in general, that when the attention of the Mind is deeply engag’d to any particular object or sensation, and either neglects or is restrain’d from exerting its influence over the organs of respiration, the Blood is stop’d in the Lungs, so that it becomes necessary to draw in a large Chestful of air, in order to give the Blood a free passage from the right Ventricle of the Heart to the left. Hence Melancholy persons, profound Mathematicians, and fond pining Lovers, are most subject to that affection. Such people are also very liable to many Hypochondriac and Chronic Diseases; which often proceed from a defective respiration, or a too slow motion of the Blood through those parts which are agitated by the alternate dilatation and contraction of the Thorax. Hence the Liver and Spleen and the Lungs themselves must suffer most when the attention of the Mind is engag’d by some Disease of its own, and it becomes less sensible of the Disorders of the Body. Hence people in Grief, &c. labour under a double Disease, which, on account of the anxiety, weight, and oppression that is felt from the Blood stagnating about the Heart, is commonly termed Heart-breaking. An Uneasiness or Anxiety, and Palpitation of the Heart, are the last Symptoms that are commonly felt of the Night-mare, which proceed from the cause lately mention’d; as ’tis then necessary that the Heart should contract itself more frequently, in order to discharge the Blood collected in the Vena Cava, the right Sinus Venosus, and Auricle, during the fit. Having done what I propos’d in this Chapter, and given the best account that I know of the Symptoms, I should now proceed to the Prognostics and method of Cure; but, as I have shewn how the vital motions are stop’d, and a general stagnation of the Blood is produc’d, it is also incumbent on me to explain how the motion of that vital stream is renew’d by the efforts of nature alone; otherwise it might be objected, that, according to my theory, unless where art interpos’d, every fit of the Night-mare would be mortal. CHAP. IV. _Of the Natural Cure._ In order to shew how persons recover out of a fit of this Disease, by the mere efforts of nature, I shall beg leave to premise a few of the most probable opinions, and best establish’d propositions, concerning Animal Motion, which I shall here take for granted, and refer the reader, for a physical demonstration of them, to the ingenious Essays of Doctors Porterfield, Whytt, Simson, and Haller. Animal and Muscular Motion is said to be of two kinds, viz. Voluntary, and Involuntary or Habitual.