The sleep of man, and of animals which have, like man, a large and well-developed nervous system—has for its salient feature the cessation or extreme lowering of the “psychical” activity of the brain. When sleep is at its height external agents (such as a touch, a sound, a flash of light) which in the waking state set up through the nerves of the organs of the senses complex changes in the brain, no longer do so. They not only fail to excite consciousness and to leave their mark on the memory, but they do not produce even a simple unconscious response. Yet if they are of a sufficient degree of violence (varying according to the depth of the sleep), they do reach the brain, and thus “awake” the sleeper. Corresponding to the absence of receptive activity of the brain in sleep is the absence of outgoing impulses from that organ; there is no such control of the muscles as in the waking state, the head nods, the eyelids droop, and the muscular action by which the erect posture is maintained is in abeyance, although in a greatly lessened degree some amount of muscular tone is unconsciously retained. The passage from the waking state to that of deep sleep is not sudden but graduated, and so is the process of awakening. In the intermediate condition, either before or after deep sleep (often only a minute or two in duration) the brain can still receive, more or less confusedly, impressions from the exterior through the organs of sense, and it is in this way that “dreams” are set going, and may be afterwards either forgotten or remembered. In full sleep the mind is a blank. As a rule healthy sleep becomes gradually more complete in the first hour, and then very slowly less profound. But there are not any sufficient observations on the “quality” of sleep after short or long duration. In sleep it is not only the brain which is at rest: the whole body shares in the condition. The pulse and breathing are slower, the digestive organs and the bladder are more or less at rest. Both the intake of oxygen into the lungs and the expiration of carbonic acid are lessened. The chemical changes within the body are lessened though still proceeding, and as a consequence the temperature is lowered.