Relationship between life and death. inside, outside: No information about this but it is two different photographs put together to create a half skull, half face image. That heavily relates to the theme of inside, outside and in between.
Michelangelo ~ Study for the drapery of the Erythraean Sibyl, brown wash with dark brown ink over a black chalk under drawing, British Museum, London. A study for one of the five Sibyls on the Sistine Chapel ceiling.