this. The position of the sleeping person is often changed unconsciously when uneasy, and the feet, when chilled, will be drawn under the bedclothes. He mentions still more impressive facts, such as sleeping on horseback, without danger of losing seat, once the habit is acquired. We may add that men sleep leaning against a wall or a tree, or even astride of a limb, without serious risk of falling. Monkeys, opossums, and other animals sleep hung up by helices formed of their tails. Somnambulism may be referred to the same class of facts.
Prof. Michael Foster, of the University of Cambridge, England, a very eminent physiologist, says, in a work published in 1891, that although the phenomena of sleep are largely confined to the central nervous system, and especially to the hemispheres of the brain, the whole body shares in the condition. "The pulse and breathing are slower. The intake of oxygen and the output of carbon dioxide, especially the latter, are lessened. Indeed, the whole metabolism" (by which term he designates generally the chemical transformations in the living organism) "and the dependent temperature of the body are lowered." (The latter statement is not in accord with other authors.) "We can not say as yet how far these are the indirect results of the condition of the nervous system, or how far they indicate a partial slumbering of the several tissues." Prof. Foster distinctly declines to enter into what he calls "the psychology of sleep and dreams." But to ignore dreams and their essential relations to natural sleep suggests to the present writer the play of Hamlet with the Prince of Denmark left out.
Foster points out the fact, as the result of all experience, that the recumbent posture is highly promotive of natural sleep, and connects it with the reduction of energy consumed in the circulation, especially with the large diminution of work that falls upon the heart. The most remarkable and significant statements made by this high authority are that "during sleep the pupil is constricted, during deep sleep exceedingly so, and dilatation, often unaccompanied by any visible movements of the limbs or body, takes place when any sensitive surface is stimulated; and on awaking, the pupils also dilate." He adds that this "contraction of the pupils is worthy of notice, since it shows that the condition of sleep is not merely the simple and direct result of the falling away of afferent" (external) "impulses. When the eyes are closed in slumber, the pupils ought, since the retina is then quiescent, to dilate; that they are constricted, the more so the deeper the sleep, shows that important actions in the brain are taking place." In other words, as we have already set forth, certain inherent functions of the brain, also of the nervous system, emotional and imaginative processes, among others, independent of what he calls