It is remarkable, that this Disorder attacks People only in sleep; which, Doctor Young says[27], is owing to the effect that sleep has in increasing all the symptoms of a Plethora. It is true, that sleep retards the motion of the Blood, and checks the serous secretions. “[28].In vasis vero serosis, Lymphaticis et Nervosis circulatio parva, et sæpe nulla est.” There is no occasion to go about proving that the secretion of urine is lessened in bed, for common experience sufficiently evinces it. And it appears, by the experiments of Doctor Robinson[29] and Gorter[30], that perspiration is considerably less in the night than in the day. It must be allow’d, that the heat of the bed-cloaths will rarify the Blood, and also contribute to an universal distension of the Vessels: but all these seem to be rather the effects of lying quiet in a warm bed, than of sleep alone. If so, People might be as readily seiz’d with the Night-mare while they are awake in these circumstances, as when they are asleep, which never happens. I really can find no way of accounting for this Phænomenon, unless we have recourse to the Soul, or that active principle within us, whose operations, during sleep, are either greatly impeded, or altogether suspended. It is therefore less sensible of any uneasiness in the Body than when we are awake, and the faculties of the Mind are in action, which is compell’d, by some innate necessity, to avoid any pain, as soon as it perceives it in the Body. While we are awake, lie on our Backs, and feel any uneasiness in that position, we immediately alter it: but, in sleep, we are not so soon conscious of the Blood’s stoppage in the Lungs, nor have we the means of removing that dangerous obstruction so much in our power, because the voluntary motions are then suspended, without which, the position of the Body cannot be changed, nor the cause of the obstruction remov’d. The insensibility of the Lungs too may contribute to render the obstruction greater, before the Mind becomes conscious of it; for we don’t find, that obstructions and inflammations of the Lungs are attended with such an acute pain, as when these Disorders attack other parts of the Body, the Liver, Spleen, and Omentum excepted. The Night-mare may sometimes seize very plethoric Persons, when they don’t lie directly on the Back; for part of the Heart’s weight may fall on the Pulmonary Veins, in a lateral position of the Body. By way of a brief recapitulation of what has been offer’d concerning the Causes in general of this Disorder, I shall conclude this Chapter with the following corollaries. COR. 1. That they who have a very sensible system of Fibres, and are soon affected by a stimulus, are least subject to the Night-mare. COR. 2. That sluggish, inactive constitutions are most liable to it. COR. 3. That the severity of the fit will be always proportional to the sensibility of the Fibres, and the quantity of Blood.