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I./1.4.: Fourth ventricle
I./1.4.1.: Sagittal section
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The fourth ventricle is a cavity formed between the pons, medulla oblongata and the cerebellum. On the top it communicates through the aquaeductus cerebri with the third ventricle. In mediansagittal section the ventricle is triangle-shaped, the base of which is formed by the brainstem, its legs (actually the roof of the ventricle) are formed by the cerebellum and two thin brain tissue lamina. The vertex angle of the triangle, meeting point of the two brain tissue lamina is called the fastigium.
I./1.4.2.: The base of the ventricle: fossa rhomboidea
When we open the ventricle from behind by removing the cerebellum, its rhomboid-shaped bottom, the fossa rhomboidea unfolds. The fossa rhomboidea is divided approximately in the middle by slightly upward-bulging brain tissue stripes, the striae medullares ventriculi quarti into two parts: the upper part, corresponding to the dorsal surface of the pons, and the lower part, the medulla oblongata’s dorsal surface. A deep trough, the sulcus medianus runs through in the midline, accompanied by the eminentia mediana on both sides. The latter is bordered by the sulcus limitans in lateral direction. In the fossa’s part above the stria medullares, the roundish protrusion of the eminentia’s, the colliculus facialis can be found. It is named after the inner knee (genu) of the nervus facialis that bypasses the nervus abducens’s nucleus. On the opposite side of the sulcus limitans, a bit upwards there is a vascular, blueish area, the locus coeruleus. Downward from this, a triangle-shaped, widening area, the area vestibularis
is located, corresponding to the vestibular nuclei. In the lateral corner of the ventricle, next to the above mentioned area, there is the tuberculum acusticum.
Under the stria medullares the fossa rhomboidea can be further segmented, according to the dorsal surface of the medulla oblongata. On both sides of the sulcus medianus the triangle-shaped trigonum nervi hypoglossi can be found. To the lateral and caudal direction from here the trigonum nervi vagi (ala cinerea) is located, which is laterally separated from the area postrema by the funiculus separans.
The lateral border of the fossa rhomboidea is formed by the lower cerebellar peduncle (pedunculus cerebellaris inferior seu corpus restiforme). The lower medial part between the sulci limitans and the striae medullares is sometimes referred to as calamus scriptorius, due to its shape resembling a fountain-pen. The ventricle here transitions into the spinal cord’s canalis spinalis. The dilated section above this used to be called ventriculus Arrantii.
I./1.4.3.: The top of the ventricle
The roof of the fourth ventricle faces backwards, due to the ventricle’s vertical (rostrocaudal) location. Its upper part is formed by the two upper pedincle (pedunculus cerebellaris superior seu brachium conjunctivum) and the velum medullare superius stretched between them. The velum is covered by the upper part of the vermis from the outside, and in the midline the frenulum velum medullare superius spreads here from the tectum mesencephali. The lower part of the roof is formed by the velum medullare inferius stretched between the two inferior cerebellar peduncles, hidden by the lower part of the vermis (nodulus) and the two cerebellar tonsils. The two laminae meet in an angle, forming the fastigium.
I./1.4.4.: Cavity system connections
The connections of the fourth ventricle with the neighbouring (inner) CSF spaces are formed by its transition into the aquaeductus cerebri and the canalis centralis.
At the same time the CSF produced in the ventricular system gets out to the subarachnoid cisterns (outer CSF space) through a single midline and two double, lateral apertures. The former is called apertura mediana ventriculi quarti or foramen Magendie, while the latter are aperturae lateralis ventriculi quarti or foramina Luschkae. The foramen Magendie opens into the cisterna magna at the lower angle of the ventricle, the obex. (The obex is an indentation between the two tuberculum gracile et cuneati). The foramina Luschkae open into the cerebellum-pons angle on both sides in the most lateral points of the fossa rhomboidea.
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