Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.2.4

7.5.2.4

tānpurastātpratīca upadadhāti | etadvai yatraitānprajāpatiḥ paśūnālipsata ta ālipsyamānā udacikramiṣaṃstānprāṇeṣu samagṛhṇāttānprāṇeṣu saṃgṛhya purastātpratīca ātmannadhatta

He puts them in the fore-part, so as to look towards the back (west). For when, on that (former) occasion, Pragâpati wanted to slaughter these animals, they, being about to be slaughtered, wanted to run away. He seized them by (the organs of) the vital airs; and having seized them by the vital airs, he took them into himself from the front (mouth) towards the back (inside).