Shatapatha Brahmana 6.5.4.15

6.5.4.15

athaināmācṛṇatti | sthemne nvevātho karmaṇaḥ prakṛtatāyai yadvevācūṇatti śira etadyajñasya yadukhā prāṇaḥ payaḥ śīrpastatprāṇaṃ dadhātyatho yoṣā vā ukhā yoṣāyāṃ tatpayo dadhāti tasmādyoṣāyām payaḥ

He then pours (milk) into it,--just for strength, or to (mark) the progress of the work. And, again, why he pours (milk) into it,--that fire-pan is the head of the sacrifice, and milk is breath: he thus lays breath into the head. Moreover, the fire-pan (ukhâ, f.) is a female: he thus lays milk into the female, whence there is milk in the female.