Third Ventricle
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Direct answer
A narrow midline slit inside the diencephalon, the third ventricle connects the two lateral ventricles through the interventricular foramina of Monro in front and drains through the cerebral aqueduct into the fourth ventricle behind. Its lateral walls are the two thalami, joined in most brains by the massa intermedia, with the hypothalamus forming the lower anterior wall; the roof carries a double layer of choroid plexus, and the floor descends through optic chiasma, infundibulum, tuber cinereum and mammillary bodies from before backwards. Five recesses — supraoptic, infundibular, pineal and suprapineal, plus the optic recess — project into surrounding structures and are the small print of neuroradiology reports.
What you must remember
- Connections: interventricular foramina of Monro with each lateral ventricle, bounded anteriorly by the fornix column and posteriorly by the thalamus; the cerebral aqueduct (of Sylvius) runs down to the fourth ventricle.
- Lateral wall: thalamus above and hypothalamus below, separated by the hypothalamic sulcus; the massa intermedia (interthalamic adhesion) bridges the cavity in about three-quarters of brains — grey matter, not a true commissure.
- Roof: ependyma with the two-layered tela choroidea through which choroid plexus invaginates, supplied by the medial posterior choroidal arteries.
- Floor, anterior to posterior: optic chiasma and recess, the infundibulum with its recess into the pituitary stalk, tuber cinereum, mammillary bodies, and posterior perforated substance.
- Anterior wall: lamina terminalis, anterior commissure and the diverging columns of the fornix; the posterior wall is marked by the pineal and habenular commissures with the pineal recess between them, and the suprapineal recess above.
- Choroid plexus strategy: the plexus of the third ventricle is continuous through the foramina of Monro with that of the lateral ventricles — one paired, roof-hugging system.
- Colloid cyst: arises at the foramen of Monro; a ball-valve obstruction of both lateral ventricles, famous for headaches that intensify on leaning forward and can cause sudden death.
- Aqueduct stenosis: congenital or acquired narrowing produces non-communicating hydrocephalus of all three ventricles proximal to it, with the third ballooning against the hypothalamus.
The ventricle as a map of the diencephalon
Draw the third ventricle in sagittal section and you have drawn the diencephalon labelled. Start below and in front: the optic chiasma forms the floor's first landmark, and a craniopharyngioma or suprasellar aneurysm here compresses chiasmal fibres from below. Behind it the infundibulum dips — the recess that makes pituitary surgery risk CSF leak — then the tuber cinereum and the mammillary bodies, targets of thiamine deficiency in Wernicke encephalopathy. The lateral wall splits the thalamus above from the hypothalamus below along the hypothalamic sulcus, and the massa intermedia crosses the midline as a landmark radiologists measure when hydrocephalus thins the thalami. Obstruction at either foramen of Monro dilates one lateral ventricle; a colloid cyst at the junction blocks both — headaches that appear on bending and vanish on standing are the classical story, and the cyst is reachable endoscopically through the foramen. Posteriorly the aqueduct is the bottleneck: stenosis or a pineal tumour lodged over it dilates the third ventricle until its floor balloons — a third ventriculostomy deliberately opens this thinned floor into the interpeduncular cistern to bypass the obstruction.
Where students slip
Recess order and ownership get scrambled: supraoptic (above the chiasma), infundibular (into the stalk), pineal (into the pineal stalk between the two commissures) and suprapineal (above the pineal). The massa intermedia is the second trap — it is an adhesion of grey matter, not a true commissure, and calling it one is marked wrong.
Frequently asked questions
Which structures form the floor of the third ventricle?
From before backwards: the optic chiasma, infundibulum, tuber cinereum, mammillary bodies and the posterior perforated substance. Each is a clinical target — chiasmal tumours, pituitary stalk surgery, hypothalamic syndromes and Wernicke encephalopathy respectively.
What is the massa intermedia?
A bridge of grey matter connecting the two thalami across the third ventricle, present in about three-quarters of people. It is an adhesion rather than a true commissure and has no major established function.
Which recesses does the third ventricle show?
The optic and supraoptic recesses around the chiasma, the infundibular recess into the pituitary stalk, the pineal recess between the habenular and posterior commissures, and the suprapineal recess. They are standard reference points in midline imaging.
Why does a colloid cyst cause episodic headaches?
The cyst sits at one or both foramina of Monro; head position can wedge it against the outflow, suddenly blocking both lateral ventricles. Bending forward classically provokes severe headache, relieved when the head position changes.
What happens when the cerebral aqueduct is stenosed?
CSF cannot reach the fourth ventricle, so the lateral and third ventricles dilate — a non-communicating hydrocephalus. In infants it presents with an expanding head and bulging fontanelles; endoscopic third ventriculostomy is a standard surgical answer.