Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.16

8.6.2.16

te dakṣiṇataḥ svayamātṛṇāyā upadadhāti | atha prathamāyāṃ citā uttararaḥ svayamātṛṇāyā udaraṃ ca yoniṃ copadadhāti yo vā ayam madhye prāṇastadeṣā svayamātṛṇaitasya tatprāṇasyobhayata udaraṃ ca yoniṃ ca dadhāti tasmādetasya prāṇasyobhayata udaraṃ ca yoniśca

He places them south of the naturally-perforated (brick). Now, in the first layer, he places both the belly and the womb north of the naturally-perforated one; for that naturally-perforated one, indeed, is what this vital air in the middle (of the body) is: he thus places the belly and the womb on both sides of that (central) vital air, and hence the belly and the womb are on both sides of that central vital air.