Śatapatha Brāhmaṇa ("of a hundred paths") 8.3.4.1

8.3.4.1

atha vālakhilyā upadadhāti | prāṇā vai vālakhilyāḥ prāṇānevaitadupadadhāti tā yadvālakhilyā nāma yadvā urvarayorasambhinnam bhavati khila iti vai tadācakṣate vālamātrādu heme prāṇā asambhinnāste yadvālasātrādasambhinnāstasmādvālakhilyāḥ

He then lays down the Vâlakhilyâs;--the Vâlakhilyâs, doubtless, are the vital airs: it is the vital airs he thus lays (into Agni). And as to why they are called Vâlakhilyâs,--what (unploughed piece of ground lies) between two cultivated fields is called 'khila;' and these (channels of the) vital airs are separated from each other by the width of a horsehair (vâla), and because they are separated from each other by the width of a horse-hair, they (the bricks) are called Vâlakhilyâs.