[403] How does one train thus: 'Experiencing the whole body, I shall breathe in'; how does one train thus: 'Experiencing the whole body, I shall breathe out'?
"Body": there are two bodies — the mental body (nāmakāya) and the physical body (rūpakāya). What is the mental body? Feeling (vedanā), perception, volition (cetanā), contact (phassa), and attention (manasikāra) — these are name and these are the mental body — and also those which are called mental formations (cittasaṅkhārā): this is the mental body. What is the physical body? The four great primary elements (mahābhūta) and the form derived from the four great primary elements, the in-breath and the out-breath, the sign (nimitta) and the anchoring (upanibandhanā), and also those which are called bodily formations (kāyasaṅkhārā): this is the physical body.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.