Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.3.2

8.3.3.2

yadveva candasyā upadadhāti | paśavo vai candāṃsyannm paśavo madhyam madhyamā citirmadhyatastadannaṃ dadhāti

And, again, as to why he lays down Khandasyâs,--the metres are cattle, and cattle are food, and the middlemost layer is the middle (of Agni, the altar): he thus places food in the middle (of Agni's body).