Subala Upanishad of Shukla-yajurveda, Chapter XI

Again Raikva asked Him: "O Lord! what is the seat of Ātmā which is replete with vijñāna? and how does it leave the body and pervade the universe?" To this He replied: "There is a mass of red flesh in the middle of the heart. In it, there is a lotus called dahara. It buds forth in many petals like a water-lily. In the middle of it is an ocean (samudra).^[1] In its midst is a koka^[2] (bird). In it there are four nādis. They are ramā, aramā, Icchā and punarbhava. Of these, ramā leads a man of virtue to a happy world. Aramā leads one of sins into the world of sins. (Passing) through Icchā (nādi), one gets whatever he remembers. Through punarbhava, he splits open the sheaths; after splitting open the sheaths, he splits open the skull of the head; then he splits open pṛthivī; then āpas; then tejas; then vāyu; then ākāś. Then he splits open manas; then bhūtādi; then mahat; then avyakta; then akṣara; then he splits open mṛtyu and mṛtyu becomes one with the supreme God. Beyond this, there is neither Sat nor asat, nor Sat-asat. Thus is the exposition of Nirvāṇa; and thus is the exposition of the Vedas; yea, thus is the exposition of the Vedas."