The vast oppression on the Breast, and immobility of the Body, which are always felt in this Disorder, probably arise from the quantity of Blood collected in the Lungs, Vena Cava, right Ventricle, and Auricle of the Heart; nor does the Mind appear to be mistaken in this case, as some have imagined; for it seems the same with regard to the Mind, whether the real action of the Muscles be constrain’d by a superior external force, or the influence of it over these Muscles be hinder’d by an internal cause. In a fit of the Night-mare, the Mind, conscious of the dangerous situation of the Body, in vain endeavours to alter it, because its power over the Voluntary Muscles is some way suspended, by the obstruction of the Blood; yet the Mind may exert itself as much as if it strove to remove the greatest obstacle. In this case the Mind generally ascribes the immobility of the Body to some great weight laid on the Breast; whereas the cause is really internal: and people judge of the greatness of the oppression, according to the efforts nature makes to overcome the obstruction of the Blood in the Lungs.
Besides, in heavy or profound sleep, the voluntary motions are generally stop’d. Hence, when people awake suddenly, they are for some time Paralytic, before the Animal Spirits obey the commands of the Mind, and actuate the Muscles in the usual manner.
The indistinct Voice is probably owing to the same cause; for the Muscles of the Tongue and Larynx, which form distinct sounds, are of the voluntary class, which, as was said before, are generally suspended in sleep.
The collapsing of the Lungs, which are, at this time, overloaded with Blood, will exclude the air, that necessary medium of sounds, and sole vehicle of voice.
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.