Does anybody know the meaning of the drawings on the four corners of Fra Mauro's Map?

Question

If you could send me a reasonable explanation of this map I would be very pleased.

And... by any chance can someone tell me why is it inverted?

Answer 1

" SKETCH-MAP OF FRA MAURO'S MAPPE-MONDE 304

As reduced and simplified in Lelewel's _Atlas_.The corners of the table are filled up with four small circles representing:

(1) The Ptolemaic System in the Spheres.
(2) The lunar influences over the tides.
(3) The circles described in the terrestial globe.
(4) A picture of the expulsion from Eden, with the four sacred rivers.