Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.3.31

9.2.3.31

yadvevainamabhijuhoti | śira etadyajñasya yadagniḥ prāṇaḥ payaḥ śīrṣaṃstatprāṇaṃ dadhāti yathā svayamātṛṇāmabhiprakṣaredevamabhijuhuyātprāṇaḥ svayamātṛṇā rasa eṣa śiraśca tatprāṇaṃ ca rasena saṃtanoti saṃdadhāti naktoṣāsā samanasā virūpe iti tasyokto bandhuḥ

And, again, as to why he makes offering upon it. That (Âhavanîya) fire is the head of the sacrifice, and milk means breath: he thus puts breath into the head. He should make the offering so that it (the milk) flows on the naturally-perforated (brick);--the naturally-perforated one is breath, and this (milk) is vital sap: he thus connects and puts together the head and the breath by means of vital sap. [He pours it out, with, Vâg. S. XVII, 70, 71], 'Night and Dawn, of one mind, unlike in form,'--the meaning of this has been explained.