Śatapatha Brāhmaṇa ("of a hundred paths") 8.7.3.11

8.7.3.11

yadveva vikarṇīṃ ca svayamātṛṇāṃ copadadhāti | āyurvai vikarṇī prāṇaḥ svayamātṛṇāyuśca tatprāṇaṃ copadadhātyuttame upadadhātyuttame hyāyuśca prāṇaśca saṃspṛṣṭe saṃspṛṣṭe hyāyuśca prāṇaśca pūrvāmuttarāṃ vikarṇīmupadadhātyāyuṣā tatprāṇamubhayataḥ parigṛhṇāti

And, again, as to why he lays down the Vikarnî and the Svayamâtrinnâ; the Vikarnî, doubtless, is vital power, and the naturally-perforated one is vital air: he thus bestows both vital power and vital air. He lays them down as the two last (highest bricks), because vital power and vital air are the two highest (endowments); and close together, because vital power and vital air are closely (bound) together. The upper (northern) Vikarnî he lays down first: he thereby encloses the vital air on both sides in vital power.