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.
As an artist - especially a portrait artist - you have to become very familiar with the human skull. It is the foundation of the face. If you are ever going to accurately capture someone's likeness, you need to understand what lies beneath their skin.