Fourth ventricle

Boundaries and connections

Anatomically, the cavity of the IV ventricle is bounded by vital structures of the central nervous system. Its anterior wall is formed by the dorsal surfaces of the pons and medulla oblongata, while posteriorly the cavity is covered by the mass of the cerebellum.

Cerebrospinal fluid (liquor cerebrospinalis) circulates inside the ventricle. To ensure continuous CSF flow, the ventricle communicates with adjacent spaces:

• Superiorly — transitions into the cerebral aqueduct (aqueductus cerebri), through which it connects with the III ventricle.
• Inferiorly — narrows in a funnel-like shape and continues into the central canal of the spinal cord.
• With the subarachnoid space — communicates at three sites. 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), opening in the region of the lateral recesses of the ventricle (recessus laterales ventriculi quarti).

Roof architecture

The roof of the IV ventricle has a characteristic tent-like shape arching over the rhomboid fossa. It is formed by several structural elements:

1. Superior medullary velum (velum medullare rostrale or superius) — a thin lamina tightly stretched between the right and left superior cerebellar peduncles.
2. Inferior medullary velum (velum medullare caudale or inferius).
3. Choroid tela of the IV ventricle (tela choroidea ventriculi quarti) — represents a rudimentary posterior wall of the primary brain vesicle.

Within the thickness of the tela choroidea, the villous choroid plexus (plexus choroideus ventriculi quarti) is formed, whose main task is CSF secretion. If this plexus is carefully removed on an anatomical specimen, a visible trace remains on the lateral walls of the ventricle — the taenia of the fourth ventricle (tenia ventriculi quarti).

Ventricular floor: rhomboid fossa

The floor of the IV 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 opening of the cerebral aqueduct 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 both sides of the median sulcus, the medial eminences (eminentiae mediales) rise. Gray matter clusters — motor cranial nerve nuclei — lie within their thickness. In the posterior sections of the superior triangle on these eminences, the facial colliculus (colliculus facialis) stands out, its relief 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 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 lower part of the fossa

In the lateral angles of the rhomboid fossa lies the vestibular area (area vestibularis), which serves as the projection site of the vestibular nuclei.

Lower and closer to the midline, two important triangles are distinguished:

• Hypoglossal trigone (trigonum n. hypoglossi) — occupies a more medial position.
• Vagal trigone (trigonum n. vagi) — lies somewhat lower and lateral to the previous one.

Along the edge of the vagal trigone runs the funiculus separans (funiculus separans). Caudal to both triangles lies a special posterior area — area postrema. It is unique in being covered by specialized ependyma that performs a chemoreceptor function (monitoring the chemical composition of blood and CSF).

In the most caudal (lower) 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-shaped roof of the IV ventricle.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Projection of cranial nerve nuclei onto the structures of the rhomboid fossa
• Anatomy and physiology of the area postrema chemoreceptor zone
• Mechanisms of CSF production by the choroid plexus of the IV ventricle
• Topographic connections of the locus coeruleus

Other topics in the "CNS" section

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