Shatapatha Brahmana 7.5.1.35

7.5.1.35

paśureṣa yadagniḥ | so 'traiva sarvaḥ kṛtsnaḥ saṃskṛtastasyāvāṅ prāṇaḥ svayamātṛṇā śroṇī dviyajuḥ pṛṣṭayo retaḥsicau kīkasā viśvajyotiḥ kakudamṛtavye grīvā aṣāḍhā śiraḥ kūrmo ye kūrme prāṇā ye śīrṣanprāṇāste te

Now that Agni is an animal, and even now he is (being) made up whole and complete: the naturally-perforated (brick) is his lower vital air, the dviyagus the hip, the two retahsik the ribs, the visvagyotis the breast-bone, the two seasonal ones the back, the ashâdhâ the neck, the tortoise the head, and the vital airs in the tortoise are those vital airs in the head.