Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.1.5

10.2.1.5

atha nirṇāmau pakṣayoḥ karoti nirṇāmau hi vayasaḥ pakṣayorbhavato vitṛtīye vitṛtīye hi vayasaḥ pakṣayornirṇāmau bhavato'ntare vitṛtīye'ntare hi vitṛtīye vayasaḥ pakṣayornirṇāmau bhavataḥ sa caturaṅgulameva purastādudūhati caturaṅgulam paścādupasamūhati tadyāvadevodūhati tāvadupasamūhati tannāhaivātirecayati no kanīyaḥ karoti

He then makes two bending-limbs in the wings, for there are two bending-limbs in a bird's wings. In one-third (of each wing he makes them), for the bending-limbs are in one-third of the bird's wings;--in the inner third, for the bending-limbs are in the inner third of a bird's wings. He expands (each of these limbs) in front by just four finger-breadths, and contracts it behind by four finger-breadths; he thus expands it by just as much as he contracts it; and thus, indeed, he neither exceeds (its size), nor does he make it too small.