Boundaries and Connections
Anatomically, the cavity of the fourth ventricle is bounded by vital central nervous system structures. Its anterior wall is formed by the dorsal surfaces of the pons and medulla oblongata, while its posterior wall is covered by the cerebellum.
Cerebrospinal fluid (liquor cerebrospinalis) circulates within the ventricle. To ensure continuous CSF flow, the ventricle communicates with neighboring spaces:
- Superiorly — transitions into the cerebral aqueduct (aqueductus cerebri), connecting it to the third ventricle.
- Inferiorly — tapers in a funnel-like shape and continues into the central canal of the spinal cord.
- With the subarachnoid space — communicates at three locations. These openings lead into the large cerebellomedullary cistern. They include a single unpaired median aperture (apertura mediana ventriculi quarti), which is the largest and lies in the lower part of the roof, as well as two lateral apertures (aperturae laterales ventriculi quarti), which open in the region of the lateral recesses (recessus laterales ventriculi quarti).
Architecture of the Roof
The roof of the fourth ventricle has a characteristic tent-like shape hanging over the rhomboid fossa. It is formed by several structural elements:
- Superior medullary velum (velum medullare rostrale or superius) — a thin lamina tightly stretched between the right and left superior cerebellar peduncles.
- Inferior medullary velum (velum medullare caudale or inferior).
- Choroid tela of the fourth ventricle (tela choroidea ventriculi quarti) — a rudimentary structure representing the posterior wall of the primary brain vesicle.
Within the substance of the choroid tela, the villous choroid plexus (plexus choroideus ventriculi quarti) develops, with its primary function being CSF secretion. If this plexus is carefully removed during anatomical dissection, a visible band remains along the lateral walls of the ventricle — the taenia of the fourth ventricle (tenia ventriculi quarti).
Floor of the Ventricle: Rhomboid Fossa
The floor of the fourth ventricle is called the rhomboid fossa (fossa rhomboidea). This diamond-shaped depression is located on the dorsal surface of the brainstem (pons and medulla oblongata) and extends from the cerebral aqueduct opening down to the entrance of the central canal.
The geometry of the fossa is strictly symmetrical:
- The long diagonal is formed by the median sulcus (sulcus medianus), which divides the floor into symmetrical right and left triangles.
- The short diagonals run transversely between the lateral recesses of the ventricle.
On either side of the median sulcus lie the medial eminences (eminentiae mediales). These contain collections of gray matter — the motor nuclei of the cranial nerves. In the posterior part of the superior triangle on these eminences stands out the facial colliculus (colliculus facialis), whose relief is formed by the internal genu of the facial nerve.
Laterally, the medial eminences are bounded by the sulcus limitans (sulcus limitans). Along this sulcus, two depressions are noticeable: the superior fovea (fovea rostralis/superior) and the inferior fovea (fovea caudalis/inferior). In the upper parts of the sulcus limitans, a specifically pigmented area shines through — the locus coeruleus (locus coeruleus).
Areas and Triangles of the Inferior Fossa
In the lateral angles of the rhomboid fossa lies the vestibular area (area vestibularis), which serves as the projection site for the vestibular nuclei.
Inferiorly and closer to the midline are two important triangles:
- Trigone of the hypoglossal nerve (trigonum n. hypoglossi) — occupies a more medial position.
- Trigone of the vagus nerve (trigonum n. vagi) — lies somewhat inferior and lateral to the previous one.
Along the margin of the vagal trigone runs a distinct bundle — the funiculus separans (funiculus separans). Caudal to both triangles lies a specialized posterior area — the area postrema. It is unique because it is covered by specialized ependyma that performs chemoreceptor functions (monitoring the chemical composition of blood and CSF).
In the most caudal (inferior) part of the rhomboid fossa, the median sulcus smoothly transitions into the central canal of the spinal cord. The entrance to this canal is partially covered by the obex — the lower margin of the tent-like roof of the fourth ventricle.