Brihadaranyaka-Upanishad: First Adhyâya, First Brahmana
1. Verily the dawn is the head of the horse which is fit for sacrifice, the sun its eye, the wind its breath, the mouth the Vaisvanara fire, the year the body of the sacrificial horse. Heaven is the back, the sky the belly, the earth the chest, the quarters the two sides, the intermediate quarters the ribs, the members the seasons, the joints the months and half-months, the feet days and nights, the bones the stars, the flesh the clouds. The half-digested food is the sand, the rivers the bowels, the liver and the lungs the mountains, the hairs the herbs and trees. As the sun rises, it is the forepart, as it sets, the hindpart of the horse. When the horse shakes itself, then it lightens; when it kicks, it thunders; when it makes water, it rains; voice is its voice.
2. Verily Day arose after the horse as the (golden) vessel, called Mahiman (greatness), which (at the sacrifice) is placed before the horse. Its place is in the Eastern sea. The Night arose after the horse, as the (silver) vessel, called Mahiman, which (at the sacrifice) is placed behind the horse. Its place is in the Western sea. Verily, these two vessels (or greatnesses) arose to be on each side of the horse.
As a racer he carried the Devas, as a stallion the Gandharvas, as a runner the Asuras, as a horse men. The sea is its kin, the sea is its birthplace.