Shatapatha Brahmana 6.2.1.24

6.2.1.24

ubhayīrgāyatrīśca triṣṭubhaścānvāha | prāṇo gāyatryātmā triśṭupprāṇamevāsya gāyatrībhiḥ saminddha ātmānaṃ triṣṭubbhirmadhye triṣṭubho bhavantyabhito gāyatryo madhye hyayamātmābhitaḥ prāṇā bhūyasīḥ purastādgāyatrīranvāha kanīyasīrupariṣṭādbhūyāṃso hīme purastātprāṇāḥ kanīyāṃsa upariṣṭāt

He recites both gâyatrî and trishtubh verses; for the gâyatrî metre is the vital air, and the trishtubh is the body (self): by the gâyatrî verses he thus kindles his vital air, and by the trishtubh ones the body. The trishtubh verses are in the middle, and the gâyatrî verses on both sides thereof; for this body is in the middle, and the (organs of) the vital airs are on the sides thereof. He pronounces more gâyatrî verses before, and fewer after (the trishtubh verses); for there are more (organs of the) vital airs in front, and fewer behind.