An Essay on the Incubus, or Night-mare

by M.D. John Bond

HAving mentioned the most remarkable opinions, that have occurr’d to me concerning the cause of this Disease, and shewn them all defective, I shall next consider several circumstances attending an horizontal position of the Body in sleep, in which alone this disease is felt; and endeavour from thence to investigate the real cause of it.

Sleep is the balmy anodyne of nature; and was intended, by the all-wise Author of our being, to ease the toils of the body, dispel the cares of the mind, and to repair the losses sustain’d by the fatigue of the day. In it we see every external stimulus remov’d, the Senses lock’d up, and every Muscle relax’d, except the Heart, the Sphincters, and those concern’d in respiration. Nutrition is then principally perform’d, and then only the Fluids glide equably through the Vessels.

As many of the voluntary Muscles are imployed in keeping the Body erect, ’tis necessary that the Body should be in an inclin’d or horizontal position, in order to relax them, and promote the salutary end of sleep. Accordingly we find, that most of the brute, as well as the human Species, chuse some easy posture of this kind to sleep in.

When the human Body lies horizontally, the Blood must flow in greater abundance to the Head; and with a greater momentum, cæteris paribus, through the Carotid and Vertebral Arteries, than when the Body stands erect; because the Blood, moving through these tubes in an horizontal direction, will not so much resist the force of the Heart, as when it ascends perpendicularly contrary to its own gravity.

No one, I presume, will doubt the truth of this proposition, who reflects, that it is much easier to move any spherical body on an horizontal plane, than to raise the same body up against a perpendicular wall.

Neither will it be denied, that the quantity as well as the velocity of Blood, flowing into the Carotid and Vertebral Arteries, is increas’d by the horizontal position of the Body, if it be consider’d, that these tubes (particularly the left Carotid) arise from, and proceed almost parallel with the axis of the Aorta, where the velocity of the Blood rushing out of the Heart is greatest. Whence it follows, from Sir Isaac Newton’s second general law of motion, and from a well known axiom in hydraulics, that these Arteries must receive more Blood in the same time, than any other branches of the Aorta of the same diameter.

As the Blood must lose most of the motion which it receives from the Heart, in passing through the infinite vascular ramifications, and fine filtres of the Brain, there scarce appears, even in an erect position of the Body, any propelling power to push it back again to the Heart, except we admit the pulsation of the small Arteries belonging to the coats of the Sinuses, and its own gravity. But in an horizontal position, the Blood has not the advantage of its gravity to accelerate its motion through the Jugular Veins; therefore it must move slower, and must be more subject to obstruction in the vessels of the Brain. Hence we see the use of pillows is to promote and facilitate the return of the Blood through the Jugular Veins: hence we may also observe, the uneasiness and danger attending the too common method of making the feet of beds higher than the heads, since a stoppage of the Blood is always productive of dangerous consequences; of which any one may be soon convinc’d by stooping the Head for a short time; and it will appear, that the Blood is by this means collected in the Veins of the Face, which will produce a Vertigo, and, if long continued, may bring on an Apoplexy. Hence we sometimes hear of people dropping down dead, upon stooping to buckle their shoes. These instances should deter some from putting their pillows under their feet, in order to make the Blood settle in their faces, and to decorate the external part of their Heads at the expence of the internal.

Notwithstanding the inconveniences and bad effects which may arise from the Blood’s delay in the Brain, yet, its being sent to the Head in sleep in a greater quantity, may serve many necessary purposes, and render sleep more beneficial and refreshing to animals. First, by distending the Blood-vessels of the Cerebrum, increasing the pressure on that part, and by that means producing sleep. Secondly, by promoting the secretion, and preparing a store of animal spirits to supply the expence of the ensuing day. Thirdly, by gently encreasing the pressure of the Blood-vessels on the Cerebellum, and perhaps determining a greater quantity of the nervous influence to the Heart, respiratory Muscles, and other parts, whose Nerves spring from that fountain of life. This pressure on the Cerebellum may concur with the rarefaction of the fluids, to render the motions of these organs more regular and vigorous in sleep.

To this mechanical pressure on the Cerebellum, the illustrious Van Sweiten seems to attribute the motion of the Heart: “Cerebelli enim actio in Cor per Nervos, pendet ab ipsa actione Cordis per Arterias[8].”