Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.15

3.8.3.15

sa hṛdayasyaivāgre 'vadyati | tadyanmadhyataḥ sato hṛdayasyāgre 'vadyati prāṇo vai hṛdayamato hyayamūrdhvaḥ prāṇaḥ saṃcarati prāṇo vai paśuryāvaddhyeva prāṇena prāṇiti tāvatpaśuratha yadāsmātprāṇo 'pakrāmati dārveva tarhi bhūto 'narthyaḥ śete

He first makes a portion of the heart. The reason why he first makes a portion of the heart which is in the middle, is that the heart is the breath, since it is from there that this breath moves upward; and the animal is breath, for only so long does the animal (live) as it breathes with the breath; but when the breath departs from it, it lies there useless, even (as) a block of wood.