Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.13

8.3.2.13

tā vā etāḥ | catasra ṛtavyāstāsāṃ viśvajyotiḥ pañcamī pañca diśyāstaddaśa daśākṣarā virāḍannaṃ virāṇmadhyam madhyamā citirmadhyatastadannaṃ dadhāti tā anantarhitāḥ svayamātṛṇāyā upadadhāti prāṇo vai svayamātṛṇānantarhitaṃ tatprāṇādannaṃ dadhātyuttarā uttaraṃ tatprāṇādannaṃ dadhāti

There are here four Ritavyâs, the Visvagyotis being the fifth, and five Disyâs,--this makes ten: the Virâg consists of ten syllables, and the Virâg is food, and the middlemost layer is the middle;--he thus puts food in the middle (of the body). He lays them down so as not to be separated from the naturally-perforated one, for the naturally-perforated one is the vital air: he thus places the food so as not to be separated from the vital air. Subsequently (to the central brick) he lays them down: subsequently to (or upon) the vital air he thus places food.