This is a scholarly annotated translation of the opening passage of the Brihadāranyaka Upanishad (Part 2 of Max Müller's 1879 Sacred Books of the East series), a foundational text of Hindu Vedantic philosophy. The passage presents the Ashvamedha (horse sacrifice) as a cosmic allegory, systematically mapping parts of the sacrificial horse's body onto elements of the universe: its head is the dawn, its eye the sun, its breath the wind, its body the year, its bones the stars, and so forth. A distinctive claim of the text is that the horse itself is a microcosm of all existence — encompassing sky, earth, seasons, rivers, and mountains — while its behaviors (shaking, kicking, urinating) correspond to lightning, thunder, and rain. The passage also assigns the horse a mythological role as carrier of different cosmic beings: the Devas, Gandharvas, Asuras, and humans, each knowing it by a different name. Müller's extensive footnotes locate the passage within the Shatapatha Brāhmana tradition, flag textual variants between the Mādhyandina and Kānva recensions, and wrestle with philological puzzles such as the obscure term *pāgasya* and the meaning of *klomāna*. Readers interested in Vedic ritual cosmology, comparative mythology, or the history of Sanskrit scholarship will find this a dense but foundational primary source.