Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.1.38

7.5.1.38

udaramukhā | yonirulūkhalamuttarokhā bhavatyadharamulūkhalamuttaraṃ hyudaramadharā yoniḥ śiśnam musalaṃ tadvṛttamiva bhavati vṛttamiva hi śiśnaṃ taddakṣiṇata upadadhāti dakṣiṇato vai vṛṣā yoṣāmupaśete yadu paśoḥ saṃskṛtasyānnaṃ taddūrveṣṭakā tasya vā etasyottaro 'rdha udāhitataro bhavati paśureṣa yadagnistasmātpaśoḥ suhitasyottaraḥ kukṣirunnatataro bhavati

The fire-pan is the belly, the mortar the womb;--the fire-pan is above, and the mortar below; for the belly is above, and the womb below. The pestle is the sisna; it is round-like, for the sisna is round-like. He places it to the right (south of the mortar), for the male lies on the right side of the female. And what food there is for the consecrated animal, that is the dûrvâ-brick. The left (north) side of that (Agni or altar) is more raised,--that Agni is an animal, and hence the left side of the belly of a well-filled beast is more raised (than the right side).