He says, “Si supine dormiant, Ventriculus ille quartus, Lympha nimium distensus, Medullam Oblongatam sua gravitate premit, ideoque fluxum liquidi Nervosi in Nervis cordi & respirationi inservientibus impedit[6].” Perhaps he did not apply his first observation so well as might be expected from one of his abilities; for it seems needless to wait for a slow secretion of Lymph to produce this Disease, since, according to his own account, the return of the Blood from the Head, by the Jugular Veins, is in some measure prevented, and by that means a greater quantity of Blood than usual will be collected in all the vessels of the Brain; which might better answer his purpose, and more effectually obstruct the nervous influence. But before either of these causes could be removed by common methods, life would be at an end, and every fit of the Night-mare would be mortal; but that it often happens otherwise, many can testify. Doctor Lower seems to have founded this theory on the dissection of a Man who died of a Hydrocephalus, and not immediately of the Night-mare: hence that case is ill applied by Bonetus[7]. CHAP. II. _An enquiry concerning the real cause of the Night-mare._ 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.