Last revised by Yuranga Weerakkody on 4 Feb 2021

The obex is the most caudal point within the fourth ventricle, as it narrows and communicates with the central canal of the spinal cord. It is one of four routes via which cerebrospinal fluid may flow from the fourth ventricle (the others being the median and two lateral apertures).

The obex occurs at the level of the foramen magnum, with anatomical studies demonstrating the obex is approximately 10-12 mm above the plane of the foramen magnum. However in patients with with low tonsillar position, such as Chiari II malformation, the obex was below the plane of the foramen magnum3.

The obex is a reliable and consistent measure of the cervicomedullary junction, therefore, it also represents the level which distinguishes the intracranial medulla and the extracranial spinal cord.

History and etymology

"Obex" is Latin for "barrier".

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Cases and figures

  • Fig 1: Gray's anatomy illustration
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  • Fig 2: Gray's anatomy illustration
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