The cranial nerves are traditionally grouped together with a total number of 12. This is not completely correct, because the olfactory system (cranial nerve I) and the optic nerve (cranial nerve II) should be considered as embryologic evaginations of fibre tracts from the telencephalon and diencephalon, and therefore are not cranial nerves in the true sense of the word. This embryological difference is anatomically reflected in a different covering of the nerve and pathologically in different disease processes and tumours, which do not affect the other cranial nerves.