Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.3.27

3.8.3.27

atha yadakṣṇayāvadyati | savyasya ca doṣṇo dakṣiṇāyāśca ca doṣṇaḥ savyāvāśca śroṇestasmādayam paśurakṣṇayā pado haratyatha yatsamyagavadyetsamīco haivāyam paśuḥ pado harettasmādakṣṇayāvadyatyatha yanna śīrṣṇo 'vadyati nāṃsayornānūkasya nāparasakthayoḥ

And because he cuts crossways,--of the left fore-foot and the right haunch; and of the right forefoot and the left haunch,--therefore this animal draws forward its feet crossways. But were he to cut straight on, this animal would draw forward its feet (of the same side) simultaneously: therefore he cuts crossways. Then as to why he does not make cuttings of the head, nor the shoulders, nor the neck, nor the hind-thighs.