Shatapatha Brahmana 13.4.4.6

13.4.4.6

tadyadeta evaṃ yūpā bhavanti prajāpateḥ prāṇeṣūtkrānteṣu śarīraṃ śvayitumadhriyata tasya yaḥ śleṣmāsītsa sārdhaṃ samavadrutya madhyato nasta udabhinatsa eṣa vanaspatirabhavadrajjudālastasmātsa śleṣmaṇāḥ śleṣmaṇo hi samabhavattenaivainaṃ tadrūpeṇa samardhayati tadyatso'gniṣṭho bhavati madhyaṃ vā etadyūpānāṃ yadagniṣṭho madhyametatprāṇānāṃ yannāsike sva evainaṃ tadāyatane dadhāti

Then as to why these stakes are suchlike. When Pragâpati's vital airs had gone out of him, his body began to swell; and what phlegm there was in it that flowed together and burst forth from inside through the nose, and it became this tree, the raggudâla, whence it is viscid, for it originated from phlegm: with that form (quality) he thus endows it (the stake). And as to why it is the (stake) standing by the fire, it is because that one is the centre of the stakes, and that nose is the centre of the (channels of the) vital airs: he thus puts it in its own place.