The rhomboid fossa is a shallow and diamond-shaped depression situated on the dorsal aspects of the medulla oblongata and pons. It is so named because the fossa has a rhomboid shape. It also forms the floor of the fourth ventricle.

The rhomboid fossa contains four margins and four corners:

Two superior margins are formed by the superior cerebellar peduncles and a part of the inferior cerebellar peduncles.
The upper corner and lower corner of the rhomboid fossa are narrow, while both lateral corners are wide.
The upper corner is continuous with the cerebral aqueduct located in the midbrain. The lower corner contains the most caudal part of the fourth ventricle called the obex, which narrows and connects with the central canal of the spinal cord.
The lateral corners contain lateral recesses, which are lateral extensions of the dorsal inferior cerebellar peduncles, and these are also sites where the medulla and pons merge.

The surface of the rhomboid fossa is not smooth. It contains several landmarks. See next slides for more detail!

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Rhomboid fossa