That the respiratory element is the chief factor in dreams of flying is clearly indicated by the fact that many persons subject to such dreams are conscious on awaking from them of a sense of respiratory or cardiac disturbance. I am acquainted with a psychologist who, though not a frequent dreamer, is subject to dreams of flying, which do not affect him disagreeably, but on awaking from them he always perceives a slight flutter of the heart. Any such sensation is by no means constant with me, but I have occasionally noted it down in exactly the same words after this kind of dream.[114] It is worth while to observe, in this connection, how large a number of people, and especially very young people, associate their dreams of flying with staircases. The most frequent cause of cardiac and respiratory stimulation, especially in children, who constantly run up and down them, is furnished by staircases, and though in health this fact may not be obvious, it is undoubtedly registered unconsciously, and may thus be utilised by dreaming intelligence. There is, however, another element entering into the problem of nocturnal aviation: the state of the skin sensations. Respiratory activity alone would scarcely suffice to produce the imagery of flight if sensations of tactile pressure remained to suggest contact with the earth. In dreams, however, the sense of movement suggested by respiratory activity is unaccompanied by the tactile pressure produced by boots or the contact of the ground with the soles of the feet. In addition, also, there is probably, as Bergson also has suggested, a numbness due to pressure on the parts supporting the weight of the body. Sleep is not a constant and uniform state of consciousness; a heightened consciousness of respiration may easily co-exist with a diminished consciousness of tactile pressure due to anaesthesia of the skin.[115] In normal sleep it may, indeed, be said that the conditions are probably often favourable to the production of this combination, and any slight thoracic disturbance even in healthy persons, arising from heart or stomach, and acting on the respiration, serves to bring these conditions to sleeping consciousness and to determine the dream of flying. Dreams of flying are sometimes associated with dreams of falling, the falling sensation occurring either at the beginning or at the end of the dream; such a dream may be said to be of the Icarus type.[116] Jewell considers that the two kinds of dream have the same causation, the difference being merely a difference of apperception. The frequent connection between the two dreams indicates that the causation is allied, but it scarcely seems to be identical. If it were identical, we should scarcely find that while the emotional tone of the dream of flying is usually agreeable, that of the dream of falling is usually disagreeable.[117]