The obex is the point at the lower tip of the fourth ventricle where it narrows into the central canal of the spinal cord and medulla. It marks the bottom of the rhomboid fossa.
| Location | Dorsal medulla, at the caudal apex of the fourth ventricle between the gracile tubercles |
|---|---|
| Parts |
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| Connections |
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| Blood Supply | Posterior inferior cerebellar artery |
Landmark marking the transition from the fourth ventricle to the central canal.
It is a surgical landmark in Chiari decompression and in brainstem surgery, and its level relative to the foramen magnum is used to describe how low the brainstem sits.
Downward displacement of the cerebellar tonsils and sometimes the obex below the foramen magnum.
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