The Munsell color sphere is described on Wikpedia. But what does the inside of the sphere look like? There is some conjecture on Wikipedia (most of which is original research), and I can only find text descriptions and the one painting of the exterior that seems to pop up everywhere:
Here is the best image of the interior I could find:
It is suggestive, but doesn't show all the internal grid lines, which you would expect in a detailed schematic.
So, I had a theory that a vertical cross section of the sphere would look like this:
But a comparison with Munsell's "Color Tree" would indicate the vertical cross section should look like this:
Which of these two seem most likely to you?