Its anterior wall, formed by the back of the pons and the medulla oblongata, constitutes the floor of the fourth ventricle.

The fossa consists of three parts, superior, intermediate, and inferior: The sulcus limitans forms the lateral boundary of the medial eminence.

In the superior part of the rhomboid fossa it corresponds with the lateral limit of the fossa and presents a bluish-gray area, the locus coeruleus, which owes its color to an underlying patch of deeply pigmented nerve cells, termed the substantia ferruginea.

Winding around the inferior peduncle and crossing the area acustica and the medial eminence are a number of white strands, the striæ medullares, which form a portion of the cochlear division of the acoustic nerve and disappear into the median sulcus.

On section it is seen that the funiculus separans is formed by a strip of thickened ependyma, and the area postrema by loose, highly vascular, neuroglial tissue containing nerve cells of moderate size.