Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.2.10

8.3.2.10

yadveva catasraḥ catuṣpādā vai paśavo 'nnam paśavo madhyam madhyamā citirmadhyatastadannaṃ dadhāti

And, again, why there are four,--animals are four-footed, and animals are food; and the middle-most layer is the middle (of Agni's body): he thus puts food in the middle.