The rhomboid fossa is divided into symmetrical halves by a median sulcus which reaches from the upper to the lower angles of the fossa and is deeper below than above. On either side of this sulcus is an elevation, the medial eminence, bounded laterally by a sulcus, the sulcus limitans.

Floor of the fourth ventricle: fc: facial colliculus; ht: hypoglossal triangle; if: inferior fovea; me: medial eminence; ms: median sulcus; o: obex; sf: superior fovea; sl: sulcus limitans; sm: medullary striae of fourth ventricle; vt: vagal triangle.

In the superior part of the fossa the medial eminence has a width equal to that of the corresponding half of the fossa, but opposite the superior fovea it forms an elongated swelling, the colliculus facialis, which overlies the nucleus of the abducent nerve, and is, in part at least, produced by the internal genu of the facial nerve.

Different parts of the medial eminence are given different names according to their associated nuclear groups. Thus, caudally the hypoglossal nucleus forms the hypoglossal triangle lying close to the median plane and the dorsal nucleus of the vagus forms the vagal triangle, situated just lateral to the hypoglossal triangle.