Rudolf Steiner:
32. In the head of man, the physical organization is a copy, an impress, of the spiritual individuality. The physical and the etheric part of the head stand out as complete and self-contained pictures of the spiritual; beside them, in independent soul-spiritual existence, there stand the astral and the ego-part. Thus in the head of man we have to do with a development, side by side, of the physical and etheric, relatively independent on the one hand, and of the astral- and ego-organization on the other.
33. In the limbs and metabolic part of man the four members of the human being are intimately bound up with one another. The ego-organization and astral body are not there beside the physical and etheric part. They are within them, vitalizing them, working in their growth, their faculty of movement, and so forth. Through this very fact, the limbs and metabolic part of man is like a germinating seed, striving for ever to unfold; striving continually to become a ‘head,’ and — during the earthly life of man — no less continually prevented.
34. The rhythmic organization stands in the midst. Here the ego-organization and astral body alternately unite with the physical and etheric part, and loose themselves again. The breathing and the circulation of the blood are the physical impress of this alternate union and loosening. The inbreathing process portrays the union; the outbreathing the loosening. The processes in the arterial blood represent the union; those in the venous blood, the loosening.
Source: http://wn.rsarchive.org/GA/GA0026/English/RSP1973/GA026_index.html