How does Robert Pinsky use imagery in Song of Reasons?

In the final lines of the poem, Pinsky strings together images that glide from one to another like phantasmagoric scenes in a dream. A dog trots down the street, the street becomes "sunny pavement, plane trees . . . a flow of cars" that become "throaty music," returning the poem to the song allusion that began it. All these images, as the last line claims, are "Like the animal shapes that sing at the gates of sleep," shapes that come and go, fade in and fade out, with no reason that is readily evident.