General
- Shape and size of the foramina correlate with the size of the ventricles:
- If ventricles are small: foramen will be a crescent-shaped opening anteriorly bounded by the concave curve of the fornix and posteriorly by the convex anterior tubercle of the thalamus.
- If ventricles are large: foramen becomes more rounded.
Location
- At the junction of the roof and anterior wall of the third ventricle
Boundaries
- Anteriorly: the junction of the column and body of the fornix
- Posteriorly: anterior pole of the thalamus
Structures passing through the foramen
- Choroid plexus
- Distal branches of the medial posterior choroidal arteries,
- Thalamostriate, superior choroidal and septal veins.
Stepwise dissection from the Lateral ventricle → 3rd → Choroidal fissure →Basal cistern
A., artery; Ant., anterior; Bas., basilar; Call., callosum; Caud., caudate; Cer., cerebral; Ch., choroidal; Chiasm., chiasmatic; Chor., choroid; Col., column; Coll., colliculus; Comm., commissure; Corp., corpus; CN, cranial nerve; For., foramen; Front., frontal; Gen., geniculate; Infund., infundibular; Int., intermedia, internal; Lam., lamina; Lat., lateral; Mam., mamillary; M.P.Ch.A., medial posterior choroidal artery; Nucl., nucleus; P.C.A., posterior cerebral artery; Pell., pellucidum; Plex., plexus; Rec., recess; Sept., septal, septum; Tent., tentorial; Term., terminalis; Thal. Str., thalamostriate; V., vein; Vent., ventricle
- Superior view of the lateral ventricles.
- The choroidal fissure is the cleft between the fornix and the thalamus along which the choroid plexus is attached. The frontal horn is located anterior and the ventricular body behind the foramen of Monro.
- The thalamus forms the floor of the body of the lateral ventricle and the anterior wall of the atrium
- Enlarged view.
- The columns of the fornix form the anterior and superior margins of the foramen of Monro.
- The choroid plexus in the body extends through the posterior margin of the foramen of Monro and is continuous with the choroid plexus in the roof of the third ventricle.
- The right thalamostriate vein passes through the posterior edge of the foramen of Monro and the left thalamostriate vein passes through the choroidal fissure behind the foramen.
- Floor of the frontal horn rostrum of the corpus callosum
- Anterior wall is formed by the genu of the corpus callosum.
- Lateral wall is formed by the caudate nucleus.
- The septum pellucidum is attached to the upper edge of the body of the fornix.
- Enlarged view of the right foramen of Monro.
- The columns of the fornix form the anterior and superior margins of the foramen.
- An anterior septal vein passes backward along the septum pellucidum and crosses the column of the fornix.
- The thalamostriate vein passes forward between the caudate nucleus and thalamus and turns medially to pass through the posterior margin of the foramen of Monro to empty into the internal cerebral vein.
- The choroid plexus is attached medially by the tenia fornix to the body of the fornix and laterally by the tenia thalami to the thalamus.
- The transchoroidal exposure is begun by dividing the tenia fornix that attaches the choroid plexus to the margin of the fornix.
- The tenia thalami that attaches the choroid plexus to the thalamus is not opened.
- The opening of the choroidal fissure has been extended backward from the foramen of Monro to expose both internal cerebral veins and the medial posterior choroidal arteries coursing in the velum interpositum.
- The anterior septal vein crosses the septum pellucidum.
- The lower layer of tela choroidea, attached to the striae medullaris thalami deep to the internal cerebral veins, is intact.
The lower layer of tela choroidea that forms the floor of the velum interpositum has been opened, exposing the massa intermedia and posterior commissure within the third ventricle.
The internal cerebral veins have been separated to expose the anteroinferior part of the third ventricle.
The upper end of the midbrain forms the posterior part of the floor of the third ventricle
The mamillary bodies are situated in the midportion of the floor.
The floor anterior to the mamillary bodies and behind the infundibular recess in very thin and is the site commonly opened in a third ventriculostomy.
The chiasmatic recess extends forward above the posterior edge of the optic chiasm and below the anterior commissure
Enlarged view of the inner surface of the anterior wall of the third ventricle.
The columns of the fornix extend downward behind the anterior commissure toward the mamillary bodies.
The lamina terminalis, chiasmatic recess, posterior edge of the chiasm, and the infundibular recess are located along the anterior and lower wall of the third ventricle
The opening along the choroidal fissure has been extended posteriorly by opening the tenia fornix along the edge of the body and crus of the fornix.
The upper part of the quadrigeminal cistern, where the internal cerebral veins converge on the vein of Galen, has been exposed.
The medial posterior choroidal arteries course with the internal cerebral veins
The opening of the choroidal fissure has been extended downward along the choroidal fissure to the central part of the quadrigeminal cistern, exposing the basal and internal cerebral veins, pineal, and superior colliculus. Branches of the medial posterior choroidal arteries course beside the pineal.
Enlarged view. The tip of the pineal projects posteriorly above the superior colliculus and between the terminal part of the internal cerebral veins.
The dissection has been extended forward along the choroidal fissure toward the temporal horn by dividing the tenia on the edge of the fimbria of the fornix to expose the basal vein, posterior cerebral arteries, and trochlear nerve in the posterior part of the ambient cistern below the thalamus
The choroidal fissure in the temporal horn has been opened by dividing the tenia fimbria. The choroid plexus attachment to the thalamus has not been disturbed. The posterior cerebral artery and basal vein course through the ambient cistern on the medial side of the temporal portion of the choroidal fissure.
The exposure has been extended through the amygdala anterior to the choroidal fissure to expose the oculomotor nerve and origin of the posterior cerebral artery. The posterior cerebral artery passes above the oculomotor nerve.
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