I
Oppression on the chest has in it something so irresistible that the will cannot subdue it. A slight emotion, a little exertion, a loss of blood, or a fever is sufficient, nay, it is only necessary to enter a room where the air is warm or bad, to stoop or to go up steps, in order at once to accelerate the breath. So long as we are quiet, we may believe ourselves able to modify our respiratory movements at will; but, when the calm ceases, the working of our machine becomes apparent, and we are no longer able to arrest it. Our liberty is in no respect complete with regard to the functions of the organism. We are like children whom Nature allows to play so long as there is no danger to life.
In order to understand the meaning of the continual variations which respiration undergoes, we must remember that our body is a very complicated furnace, in which, to keep the flame of life aglow, something must constantly be burnt. The respiratory movements, the expansion and contraction of the lungs, represent the untiring work of a bellows which keeps up the fire in the smithy of our organism. We may breathe in two ways: either we expand the upper part of the thorax by raising the ribs, or we expand it lower down by depressing the diaphragm. The first movement is more common with women, in whom the rise and fall of the bosom during emotion is so characteristic; the other is more usual with men. When we sleep it is especially the diaphragm which reposes; with some persons the abdomen is almost motionless during sleep, but a slight noise or push, a voice, or any external action suffices to make it resume its functions, and the diaphragmatic breathing becomes more active. This takes place suddenly, without our waking or being aware of it, and without any recollection of it remaining in consciousness.
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.