Śatapatha Brāhmaṇa ("of a hundred paths") 7.3.1.5

7.3.1.5

yadveva vyatiṣajati | ātmā vā agniḥ prāṇo 'dhvara ātmaṃstatprāṇam madhyato dadhāti tasmādayamātmanprāṇo madhyataḥ

And, again, why he interlinks them,--Agni (the fire-altar) is the body, and the (Soma) sacrifice is the vital air: he thus places the vital air in the midst of the body, and hence that vital air is in the middle of the body.