Śatapatha Brāhmaṇa ("of a hundred paths") 8.1.4.3

8.1.4.3

sa eṣa paśuryadagniḥ | so 'traiva sarvaḥ kṛtsnaḥ saṃskṛtastasya yāḥ purastādupadadhāti tau bāhū atha yāḥ paścātte sakthyāvatha yā madhya upadadhāti sa ātmā tā retaḥsicorvelayopadadhāti pṛṣṭayo vai retaḥsicau madhyamu pṛṣṭayo madhyato hyayamātmā sarvata upadadhāti sarvato hyayamātmā

Now that Agni (the altar) is an animal, and (as such) he is even now made up whole and entire,--those (bricks) which he lays down in front are his fore-feet, and those behind are his thighs; and those which he places in the middle are that body of his. He places these in the region of the two retahsik (bricks), for the retahsik are the ribs, and the ribs are the middle, and that body is in the middle (of the limbs). He places them all round, for that body extends all round.