After this slight uneasiness, during which sleep becomes lighter for a few minutes, is past, the respiration resumes the rhythm and characteristic form of deep sleep. These changes, which take place without any participation of the will, form one of the most wonderful arrangements observable in this perfect machine of ours. When we lose consciousness, Nature could not expose our body to the influences of the external world, leaving it defenceless, a prey to dangerous enemies. It is necessary that a detachment of the nerve-centres should, even during sleep, keep watch on the outside world, and in good time prepare the material conditions of consciousness and the attitude of resistance. All unconscious phenomena which I have seen appear in the transition from sleeping to waking are destined to increase the circulation of the blood in the brain and reanimate the functions and energy of the body. These centres are sentinels on the defensive, watching continually and sounding the alarm when danger is nigh. Man falls asleep after the labours of the day. The muscles relax, head and arms fall powerless, the lids droop and cover the eyes, the legs no longer hold us erect. The feverish waking activity ceases, the fire slackens gradually within us, combustion is so much less active, that the respiratory movements, which, during calm, waking moments introduce about seven litres of air into the lungs every minute, have now reduced the ventilation to one litre per minute. The heart, too, rests by lessening the frequency of its contractions and diminishing the energy and extent of the systole; the vessels enlarge, the pressure of blood falls, and the body becomes noticeably colder. But, in spite of this loss of consciousness and complete relaxation of the body, there is still a close net of nerves and masses of nerve-cells which retain their energy and watch over us. A voice, a distant noise, a ray of light, a slight touch, any impression is enough to rouse the bellows to renewed activity, to double the number of heart-beats, to cause the vessels of the whole surface of the skin to contract, thus driving the blood to the centres of life and restoring the material conditions of consciousness. In the struggle for existence that organism will most easily escape the injuries of the external world in which this unconscious vigilance is most perfect, and which is able with the greatest promptitude to pass from the condition of profound repose to that of greatest activity before the danger comes too near and injury is inevitable. II These studies of mine of the respiration in sleeping and waking man make us understand more readily the signification of oppression during emotion. The difference is only one of intensity and degree, not of the nature and quality of the phenomenon. There is the same relation between waking and sleeping as between calm and agitation of mind. Let us consider the proof of this.