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.
COR. 4. That the duration of a fit will be proportional to the sensibility and vigour of the constitution.
COR. 5. That they who sup sparingly, and never sleep on their Backs, are seldom or never afflicted with it.
COR. 6. That it is most common in those seasons of the year, which most increase the volume of the Fluids: hence spring and autumn are its most fertile periods.