Yāska's Nirukta 13.7

13.7

``catvāri śṛṇgā trayo asya pādā dve śīrṣe sapta hastāso asya/
tridhā baddho vṛsabho roravīti maho devo martyān ā viveśa/''.
catvāri śṛṇgā iti vedā vā eta uktāh/
trayas asya pādā iti savanāni trīni/
dve śīrṣa1d prāyanīya udayanīya1d/
sapta hastāsaḥ sapta chandas1p/
tridhā baddhas tredhā baddho mantra brāhmaṇa kalpair vṛsabhas roravīti/
roravanam asya sanava kramena ṛc bhir yajus bhiḥ sāmabhir yad enam ṛc bhaiḥ śaṃsanti yajus bhir yajanti sāmabhiḥ stuvanti./
maho deva ity eṣa hi mahān devo yajñajño martyām āviveśa iti/
esa hi manuṣyān āviśati yajanāya/
tasya uttarā bhūyase nirvacanāya/

"Four are his horns, three his feet, two his heads, seven his hands;
bound in threefold manner, the bull roars loudly; the great god has entered mortals."
"Four are his horns": these are said to be the Vedas.
"Three his feet": the three pressings.
"Two his heads": the opening and concluding libations.
"Seven his hands": the seven metres.
"Bound in threefold manner": bound in three ways by the mantras, Brāhmaṇas, and manual rules, the bull roars loudly.
His roaring is sequentially by the Ṛcs, the Yajuses, and the Sāmans, because they recite over him with Ṛcs, sacrifice with Yajuses, and praise with Sāmans.
"The great god": for this great god, knowing the sacrifice, has entered mortals.
For he enters men for the sake of sacrifice.
There is a subsequent [verse] of his for further etymology: