Śatapatha Brāhmaṇa ("of a hundred paths") 9.4.3.6

9.4.3.6

tasminnaṣṭāviṣṭakā upadadhāti | aṣṭākṣarā gāyatrī gāyatro 'gniryāvānagniryāvatyasya mātrā tāvantamevainametaccinoti tāsāmaśmā pṛśnirnavamo nava vai prāṇāḥ sapta śīrṣannavāñcau dvau tānevāsminnetaddadhāti yaścite 'gnirnidhīyate sa daśamo daśa vai prāṇā madhyamāgnīdhram madhyatastatprāṇāndadhāti madhye ha vā etatprāṇāḥ santa iti ceti cātmānamanuvyuccaranti

On this (Âgnîdhrîya) he puts eight bricks,--the Gâyatrî consists of eight syllables, and Agni is of Gâyatra nature: as great as Agni is, as great as is his measure, so great he thus builds him up. The variegated stone is the ninth of them: there are nine vital airs--seven in the head and two downward ones--it is these he thus puts into it. The fire which is placed on the erected (hearth) is the tenth;--there are ten vital airs, and the Âgnîdhra is the middle (between the Gârhapatya and Âhavanîya fires).: he thus puts the vital airs in the middle of it; for the vital airs, being in the middle of the body, move along it in this direction, and in that direction.