Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.1.7

1.3.1.7

sa vā ityagrairantarataḥ sammārṣṭīti | mūlairbāhyata itīva vā ayam prāṇa itīvodānaḥ prāṇodānāvevaitaddadhāti tasmāditīvemāni lomānītīvemāni

Inside he brushes with the (grass-)tops thus (viz. from the handle to the top, or in a forward, eastward direction from himself); outside with the lower (grass-)ends thus (viz. in the opposite or backward direction, towards himself): for thus (viz. in the former way) goes the out-breathing, and thus (in the opposite way) the in-breathing. Thereby he obtains out-breathing and in-breathing (for the sacrificer): hence these hairs (on the upper side of the elbow) point that way, and these (on the lower side) point that way.