Mbh 14.24.5

"'"'Narada said, "From Sankalpa (wish) arises Pleasure. It also arises from sound. It arises also from taste; it arises too from colour. From the semen, united with blood, first flows Prana. Upon the semen being modified by Prana, flows Apana. Pleasure arises from the semen as well. It arises from taste also. This is the form (effect) of Udana. Pleasure is produced from union. Semen is generated by desire. From desire is produced the menstrual flow. In the union of semen and blood, generated by Samana and Vyana, the pair that consists of Prana and Apana, enters, moving transversely and upwards, Vyana and Samana both form a pair that moves transversely. Agni (fire) is all the deities. Even this is the teaching of the Veda. The knowledge of Agni arises in a Brahmana with intelligence. The smoke of that fire is of the form of (the attribute called) Darkness. The attribute that is known by the name of Passion is in its ashes. The quality of goodness arises from that portion of the fire into which the oblation is poured.[66] They that are conversant with sacrifices know that Samana and Vyana are from the attribute of Goodness. Prana and Apana are portions of the oblation (of clarified butter). Between them is the Fire. That is the excellent form (or seat) of Udana, as the Brahmanas know. Listen as I say which is distinct from the pairs. Day and Night constitute a pair. Between them is the Fire. That is the excellent seat of Udana as the Brahmanas know. The existent and the non-existent form a pair. Between them is the Fire. That is the excellent seat of Udana as the Brahmanas know. First is Samana; then Vyana. The latter's function is managed through it (viz., Samana). Then, secondly, Samana once more comes into operation. Only Vyana exists for tranquillity. Tranquillity is eternal Brahman. This is the excellent seat of Udana as the Brahmanas know."'"'"[67]