Complex sentence: The posterior atlantoöccipital membrane (posterior atlantoöccipital ligament), broad but thin, is connected above, to the posterior margin of the foramen magnum; below, to the upper border of the posterior arch of the atlas.
Split into simpler sentences: The posterior atlantoöccipital membrane (posterior atlantoöccipital ligament), is a broad but thin membrane. It is connected above to the posterior margin of the foramen magnum and below to the upper border of the posterior arch of the atlas.