What's Inside a Head?

As I walk around Roman Catholic cemeteries and RC plots in other cemeteries, I see many, what I describe as ceramic, portraits of Jesus and Mary affixed to headstones. On many occasions, I see what remains after they have crumbled away. I always thought they were empty but a recent visit to St Joseph's RC Cemetery in Moston, Manchester provided examples of the interiors of these heads. First a few images of more complete examples, Then some examples of the contents . . .