The glands are filled during sleep, and consequently, by drawing off from the fulness of the blood-vessels, prepare the body for vigilance, and are themselves fitted for the functions to be then performed; i.e. to excrete their proper fluids from muscular compression, or vibrations running up their excretory ducts, in the manner to be hereafter explained. The medullary substance is, in like manner, fitted and prepared for vigilance, whether it be of a glandular nature, or not. However, some vibrations must take place in it throughout, and they are particularly vivid in the regions corresponding to the heart, organs of respiration, and organs of digestion; also in the regions corresponding to the eye and ear, where they excite the trains of images which are presented to us in our dreams. But the nature of these cannot be unfolded till we have treated of ideas, their generation, and associations, and the nature of true and erroneous judgments, assent, dissent, imagination, and memory.
Cor. II. It appears also to follow, from the foregoing account of sleep, and the effect of heat, labour, pain, and opiates, in disposing to it, that, in many cases of sleep, the medullary substance tends to a subtle kind of inflammation, and is preserved from it, and restored to its natural state and degree of heat, by means of sleep sufficiently continued. Thus, in the access of most fevers, the patient is listless and sleepy, the external senses, muscles, and brain, being affected, in some respects, as by opiates. If the patient sleep, the distemper is cut short; but if the subtle inflammation be so great as to prevent that, the distemper increases, and comes to its period in some other way, according to the nature of the fever, and circumstances of the patient. In a coma vigil it seems to me, that the approach of the opposite sides of the ventricles excites such violent vibrations, on account of the inflammation of the medullary substance, perhaps of these sides particularly, as to awake the patient, and throw him into great confusion and consternation. In a phrenzy, the medullary substance itself seems to labour under an acute temporary inflammation, the other parts having often no more than a due heat, whereas, in the delirium of a fever, the medullary substance only sympathizes with the other parts. If the inflammation of the medullary substance be very subtle, moderate, and permanent, madness of some species ensues. And it seems to agree very well with the theory here proposed, that in deliriums, phrenzies, and some kinds of madness, the patient does not sleep at all, or if he do, in a quiet manner, is freed from his distemper; and that, in other kinds of madness, and in cases of melancholy, the sleep is very deep, and the patient extremely sluggish.
