Numbers 7, 9 and 40, and the Omphalos.
“Later he became fascinated with more abstract topics: numbers in Greek medicine, the numbers seven, nine and forty, and the concept of an imaginary middle point, the omphalos or world-navel, a recurrent theme in Greek, Roman and Semitic mythology.”
–W.H. Roscher, Pan and the Nightmare: Ephialtes–A Pathological-Mythological Treatise on the Nightmare in Classical Antiquity, & An Essay on Pan by James Hillman, 1972. Pp., pg. v.