Jointing Sholder shell joint

Dear friends,
I wonder if there is someone who knows the shoulder in the shape of an eggshell. Is this shell only in the form of a sphere extract or she has a thickness in its depth that raises the ball from the shoulders? I've never seen such a joint disassembled. What is the function of this shell? To cover the slot or increase the movement? How is the slot cut in such a joint? I read Zen-Anatomy of joints but it is not clear enough to me. Would anyone be good to take photo of something for reference?

 

Thank you. I think this is a very good example because it looks simple and nice. When you look at the shell, what do you think is everywhere inside the same thickness? It is not clear to me whether this is just a hollow sphere or is in its depth as in this diagram from the Zen's book more like a moon, so inside it has a greater thickness to cast more joint out (sketch in midle first on right).
Hujoo has also I see on your photos something special with hole in that shell, it is not in dead center?
l have seen photos on internet when he raises his hand it is something in that.