My own explanation was immediately suggested by the following dream. I dreamed that I was watching a girl acrobat, in appropriate costume, who was rhythmically rising to a great height in the air and then falling, without touching the floor, though each time she approached quite close to it. At last she ceased, exhausted and perspiring, and I had to lead her away. Her movements were not controlled by mechanism, and apparently I did not regard mechanism as necessary. It was a vivid dream, and I awoke with a distinct sensation of oppression in the chest. In trying to account for this dream, which was not founded on any memory, it occurred to me that probably I had here the key to a great group of dreams. The rhythmic rising and falling of the acrobat was simply the objectivation of the rhythmic rising and falling of my own respiratory muscles--in some dreams, perhaps, of the systole and diastole of the heart's muscles--under the influence of some slight and unknown physical oppression, and this oppression was further translated into a condition of perspiring exhaustion in the girl, just as men with heart disease may dream of sweating and panting horses climbing uphill, in accordance with that tendency to magnification which marks dreams generally.[111] We may recall also the curious sensation as of the body being transformed into a vast bellows or steam engine, which is often the last sensation felt before the unconsciousness produced by nitrous oxide gas.[112] When we are lying down there is a real rhythmic rising and falling of the chest and abdomen, centring in the diaphragm, a series of oscillations which at both extremes are only limited by the air. Moreover, in this position we have to recognise that the circulatory, nervous, and other systems of the whole internal organism, are differently balanced from what they are in the upright position, and that a disturbance of internal equilibrium always accompanies falling. It is also noteworthy (as, indeed, Wundt has briefly remarked) that the modifications produced by sleep in the respiratory process itself tend to facilitate its interpretation as a process of flying. Mosso showed that respiration in sleep is more thoracic than when awake, that it is lengthened, and that the respiratory pause is less marked.[113] That is to say that both the aerial element and the actual rhythmic movement of the ribs become accentuated during sleep.