The Night-mare generally seizes people sleeping on their backs, and often begins with frightful dreams, which are soon succeeded by a difficult respiration, a violent oppression on the breast, and a total privation of voluntary motion. In this agony they sigh, groan, utter indistinct sounds, and remain in the jaws of death, till, by the utmost efforts of nature, or some external assistance, they escape out of that dreadful torpid state. As soon as they shake off that vast oppression, and are able to move the body, they are affected with a strong Palpitation, great Anxiety, Languor, and Uneasiness; which symptoms gradually abate, and are succeeded by the pleasing reflection of having escap’d such imminent danger. All these symptoms I have often felt, and hope, that whoever has had, or may have, this Disease, will readily know it by this description, which I have not only taken from my own feelings, but from the observations of many of my acquaintances, who were also afflicted with it, and from the records of the antient observators. Before I enter into an enquiry concerning the cause of this Disorder, or attempt to assign any one for it myself, I shall first take notice of the principal opinions that have been advanc’d to account for it, and examine how far they are confident with the laws of the animal œconomy; that the judicious reader may see how necessary further enquiries into the nature of this Disorder may be. Doctor Willis says, That the Night-mare is owing to some incongruous matter which is mix’d with the Nervous Fluid in the Cerebellum[1]. But, as he has not told us what this matter is, or how it is produced, we can afford it little credit in this enquiry; because plethoric persons, who abound with the purest and richest Blood, in whom such incongruous matter is suppos’d least to prevail, are most subject to this Disorder[2]. Bellini, who, in many other cases, is allow’d to be a pretty accurate theorist, was strangely mistaken in this, when he said, That the Night-mare is an imaginary Disease, and proceeds from the idea of some demon, which existed in the mind the day before[3]. This account is very unworthy a physician, and is a strong evidence that he never felt the heavy effects of this Disorder; otherwise he would have allow’d it to be a real Disease of the Body. A metaphysician has laid great stress on this Disease, as an argument in defence of some of his wild opinions. He asserts, That it is owing to the operation of certains demons, which impose on, and torment, the mind in sleep[4]. This ingenious hint he took from Bellini, who probably stole it from Paracelsus’s doctrine of Archeus faber[5]. The ingenious Doctor Lower is the first author I met with, who observ’d the horizontal position of the Body, and assign’d it as a remote cause of this Disorder, but seems to attribute it immediately to a collection of Lymph in the fourth Ventricle of the Brain.