T1-weighted sagittal magnetic resonance imaging of a patient with a Chiari I malformation. The large arrowhead points to the cerebellar tonsils. The small arrowhead points to the posterior arch of the foramen magnum.
A/P: the foramen magnum allows 3 structures to pass: the brainstem, the right and left vertebral arteries (primary cerebral blood supply), and the right and left spinal roots of CN XI (shoulder shrug)