Śatapatha Brāhmaṇa ("of a hundred paths") 8.6.2.8

8.6.2.8

śroṇī jagatyaḥ | sa yāvati purastātsvayamātṛṇāyai triṣṭubha upadadhāti tāvati paścājjagatīryo vā ayam madhye prāṇastadeṣā svayamātṛṇā yāvatyu vā etasmātprāṇātpurastādurastāvati paścācroṇī

Gagatîs are the hips; at whatever distance from the naturally-perforated (central) brick he places the Trishtubhs in front, at the same distance from it he places the Gagatîs behind; for that naturally-perforated brick is this vital air in the middle (of the body), and as far from that vital air as the chest is in front, so far are the hips behind.