Notes for Select Whitman Poems

What is meant by “Song of the Universal?” Rather than using the noun universe, Whitman chooses to entitle the poem “Song of the Universal” (emphasis mine), using instead what appears to be an adjectival form of the word. Is this, however, how it is used? The OED defines universal in the modern sense, “extending over, comprehending, or including the whole of something specified or implied; prevalent over all,” but it is also shows the word being used in a manner more conducive to the reading of the poem, “Proceeding from the whole body or number; committed, given, made, etc..., by all without exception of the persons to whom there is reference or allusion.”