Shatapatha Brahmana 11.1.6.33

11.1.6.33

dvau bāhū dvā ūrū ta eva catvāraḥ patnīsaṃyājāḥ pratiṣṭhāyameva prāṇa iḍā yattāṃ nāgnau juhoti yatsāpradagdheva tasmādayamanavatṛṇaḥ prāṇaḥ

The four Patnîsamyâgas are the two arms (or front legs) and the two thighs--the support, in fact; and the Idâ is this vital air (in the centre); and inasmuch as that (Idâ) is not offered in the fire, but remains as unburnt, therefore this (central) vital air is undivided.