Shatapatha Brahmana 11.1.6.32

11.1.6.32

sa vai sakṛdavānyāt ekaṃ hyetasya parvātha yadaparvakaṃ syātpratṛṇaṃ vaiva tiṣṭhellambeta vā tasmādetaducca tiṣṭati padyate ca tasmātsakṛdavānyāt

He may, however, draw breath once, for that (organ) has one joint; but if it were jointless, it either would only stand erect, or it would hang down; whilst now it both becomes erect and hangs down: he may therefore draw breath once.