“La Main de Picasso,” at Galerie Gmurzynska New York, features just one object: a plaster sculpture of Pablo Picasso’s left hand, the only known surviving cast to exist. The gallery is narrow, and the hand is at its far end, behind velvet ropes. It is a greeting, a signal to halt, a gesture of benediction, and a relic reminiscent of the dismembered saints of old. Visit it until November 10 to reflect on the hands that made art modern.