The uncertainty of the subject, an ill-defined feeling that sleep is not altogether a physiologic affair, together with the tendency of man to be interested more in active than in passive states—these facts are assigned by Sidis as the reason for the scant attention that is given to sleep in most textbooks of physiology. The advance of biologic knowledge no longer, however, allows even passive states to be ignored.
Sidis reviews the physiologic, histologic, psychologic and biologic theories of sleep, and discusses the difference between sleep and hypnosis and the peculiar states termed by him "hypnoidal," which lie on the borderland of waking, sleep, and hypnosis. A special characteristic of the hypnoidal states is the difficulty of fixing them for any length of time. They are the bridge that connects the waking state with hypnosis on the one hand and sleep on the other. To enter or pass out of