"Cy Twombly: Treatise on the Veil" This rarely exhibited painting by Cy Twombly is accompanied by several works from the artist's private collection that "document," in a sense, the piece's creation. Treatise on the Veil comes from Twombly's "gray-ground" period (1966-1971) and very much resembles a huge chalkboard with a mysterious musical composition drawn across its lower half. The work was inspired by Pierre Henry's musique concrète composition The Veil of Orpheus, and it represents the moment in the myth when Orpheus, in an attempt to rescue his dead wife Eurydice from the underworld, breaks the rules of the gods and looks back at his wife, thus losing her forever — one of the most heartbreaking scenes in literature. That musical moment in Henry's composition is rendered as the sound of fabric tearing. Twombly, in effect, attempts to represent that sound in his own invented musical script. The piece is powerful in its size (nine feet by 32 feet) and simplicity. It's truly operatic. But the additional works on display don't necessarily support or detract from the major work. Twombly's always best when he goes big. We don't need to see the rehearsals. Just give us the performance. Through February 14. The Menil Collection, 1515 Sul Ross, 713-525-9400. — TS