# Fourth Ventricle > Fourth ventricle — rhomboid fossa landmarks, roof, foramina of Magendie and Luschka, choroid plexus and hydrocephalus for MBBS Anatomy. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/fourth-ventricle - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Fourth Ventricle", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/fourth-ventricle ## Direct answer The fourth ventricle is a tent-shaped cavity inside the pons and upper medulla, roofed by the cerebellum, whose diamond-shaped floor — the rhomboid fossa — displays landmarks for six cranial nerve nuclei. CSF enters it from the cerebral aqueduct and exits through three foramina: the median foramen of Magendie into the cerebellomedullary cistern and the two lateral foramina of Luschka into the pontine cisterns. Its roof is formed above by the superior medullary velum, stretched between the superior cerebellar peduncles, and below by the inferior medullary velum and the double choroid plexus of the tela choroidea, which invaginates through the lower roof. ## What you must remember - **Floor (rhomboid fossa):** bounded above by the superior cerebellar peduncles, below by the gracile and cuneate tubercles; divided by the median sulcus and the sulcus limitans. - **Facial colliculus:** swelling beside the midline produced by fibres of the facial nerve looping over the abducent nucleus — the nerve of one side curves around the nucleus of the sixth. - **Vestibular area:** lateral to the sulcus limitans, overlying the vestibular nuclei; swelling here with the cochlear nuclei at the lateral angle (suprafacial fossa region). - **Inferior triangle landmarks:** hypoglossal triangle medially (hypoglossal nucleus), vagal triangle lateral to it (dorsal motor nucleus of the vagus), and the area postrema beside the obex — a chemoreceptor zone linked with emesis. - **Locus coeruleus:** pigmented grey-blue at the upper end of the sulcus limitans, the noradrenaline factory of the brainstem. - **Roof:** superior medullary velum (where the trochlear nerves decussate) above; inferior medullary velum and the T-shaped choroid plexus, with its vertical limb double, below. - **Foramina:** median foramen of Magendie and lateral foramina of Luschka — the only CSF exits; blockage of all three produces a non-communicating hydrocephalus, as in Dandy–Walker malformation. - **Relations:** cerebellum above and behind, pons and medulla in front and below, cerebellomedullary cistern (cisterna magna) below — accessible in infants for cisternal puncture. ## Working through CSF exit and its failures Follow a drop of CSF from the lateral ventricle and the anatomy of hydrocephalus unfolds at the fourth ventricle. Choroid plexus of the lateral and third ventricles drives the fluid through the foramina of Monro, the aqueduct opens it into the fourth ventricle, and the three foramina release it into the subarachnoid space — the median aperture into the cisterna magna, the lateral apertures into the cerebellopontine angle cisterns. Only after this exit does CSF bathe the brain and reach the arachnoid granulations, so anything that seals the outflow traps the entire system above. In Dandy–Walker malformation the foramina fail to form: the ventricle balloons, the vermis is hypoplastic, and a posterior fossa cyst fills the expanded cisterna magna — an infant with macrocephaly and a bulging fontanelle. In tubercular meningitis, so common in Indian practice, exudate plugs the outflow foramina and basal cisterns, producing hydrocephalus that may need a ventriculoperitoneal shunt even while antitubercular therapy continues. The floor explains a different clinical entry: tumours of the medulla around the obex and the area postrema beside it explain why brainstem lesions near the fourth ventricle present with intractable vomiting. ## Where students slip The facial colliculus is the trap that never retires: it is produced by the abducent nucleus with facial nerve fibres looping over it, so a lesion here gives a lateral rectus palsy with internal squint, not facial weakness. The second error is placing the trochlear decussation vaguely "in the midbrain"; its fibres cross in the superior medullary velum, the roof of this very ventricle, and emerge below the inferior colliculi. Students also forget that the choroid plexus of the fourth ventricle is T-shaped with a double vertical limb and a single horizontal bar, and that it protrudes through the lateral foramen of Luschka into the cerebellopontine angle, where an extraventricular choroid plexus papilloma can present as a CP-angle mass. ## Frequently asked questions ### Which structures form the roof of the fourth ventricle? Superiorly the superior medullary velum between the superior cerebellar peduncles, in which the trochlear nerves decussate; inferiorly the inferior medullary velum and the tela choroidea with its T-shaped choroid plexus, overhung by the cerebellar tonsils. ### Which nuclei underlie the facial colliculus? The abducent nucleus, with the motor fibres of the facial nerve looping over it. Hence a lesion of the facial colliculus causes ipsilateral lateral rectus paralysis and internal squint. ### Through which foramina does CSF leave the fourth ventricle? The median foramen of Magendie into the cerebellomedullary cistern and the two lateral foramina of Luschka into the pontine cisterns. Obstruction of all three causes non-communicating hydrocephalus, as in Dandy–Walker malformation. ### What is the area postrema and why is it clinically relevant? A narrow strip beside the obex in the floor of the fourth ventricle, lacking a normal blood–brain barrier and functioning as a chemoreceptor trigger zone. It is central to the vomiting reflex and a target of emetic and antiemetic research. ### What is the locus coeruleus? A pigmented, grey-blue area at the upper end of the sulcus limitans in the rhomboid fossa, containing the largest cluster of noradrenergic neurons in the brain. It projects widely and regulates arousal and attention.