Śatapatha Brāhmaṇa ("of a hundred paths") 9.2.1.23

9.2.1.23

svayamātṛṇayā saṃspṛṣṭam prathamam pravargyamutsādayati | prāṇaḥ svayamātṛṇā śiraḥ pravargya ātmāyamagniścitaḥ śiraśca tadātmānaṃ ca prāṇena saṃtanoti saṃdadhātyutsādya pravargyaṃ yathā tasyotsādanam

The first Pravargya (vessel) he sets out close to the naturally-perforated (brick);--the naturally-perforated one is the breath, and the Pravargya is the head, and this built-up Agni is the body: he thus connects and puts together the head and the body by means of (the channel of) the breath. Having set out the Pravargya as is the way of its setting out,--