Shatapatha Brahmana 7.5.1.24

7.5.1.24

taṃ yatra devāḥ samaskurvan | tadasminnetatsarvam madhyato 'dadhuḥ prāṇamannamūrjaṃ tathaivāsminnayametaddadhāti retaḥsicorvelayā pṛṣṭayo vai retaḥsicau madhyamu pṛṣṭayo madhyata evāsminnetatsarvaṃ dadhāti

Now when the gods restored him (Pragâpati-Agni), they put all that inside him--breath, food, strength; and in like manner this (Sacrificer) now puts that into him. At the distance of the two retahsik (he places it),--the retahsik being the ribs, and the ribs the middle, it is thus in the middle of (or, inside) him that he puts all that.