Third (III) ventricle

The third (III) ventricle (ventriculus tertius) occupies a central position in the diencephalon. The ventricular cavity has the appearance of a sagittally located narrow slit, limited by 6 walls: two lateral, upper, lower, anterior and posterior. The lateral walls of the III ventricle are the medial surfaces of the thalami facing each other, as well as the medial parts of the subthalamic region located below the hypothalamic groove.

The lower wall, or floor, of the third ventricle is the hypothalamus, its posterior (dorsal) surface facing the ventricular cavity. In the lower wall, two protrusions (depressions) of the cavity of the third ventricle are distinguished: the recessus infundibuli and the optic recess (recessus opticus). The latter is located in front of the optic chiasm, between its anterior surface and the terminal (end) plate.

The anterior wall of the third ventricle is formed by the terminal plate, the columns of the fornix, and the anterior commissure. On each side, the column of the fornix anteriorly and the anterior part of the thalamus posteriorly limit the interventricular foramen (foramen interventriculare), through which the cavity of the third ventricle communicates with the lateral ventricle of that side.

The posterior wall of the third ventricle is the epithalamic commissure, under which is the opening of the cerebral aqueduct. In the posterosuperior parts of the third ventricle, above the epithalamic commissure, there is another protrusion of the cavity of the third ventricle - the suprapineal recess (recessus suprapinealis). All the walls of the third ventricle from the inside, from the side of its cavity, are lined with ependyma. The upper wall is formed by the vascular base (tela choroidea). This base is represented by a soft (vascular) membrane, which penetrates into the cavity of the diencephalon - into the third ventricle - with two sheets (in the form of a duplication) under the splenium of the corpus callosum and the fornix. The upper sheet of the membrane fuses with the lower surface of the fornix of the brain. At the level of the interventricular openings, this sheet is folded and, directed back, passes into the lower sheet, which is actually the roof of the third ventricle. Further back, this leaf covers the pineal body from above and lies on the upper posterior surface (roof) of the midbrain.

The upper and lower layers of the soft membrane of the brain, together with the blood vessels located in it, penetrate into the cavity of the lateral ventricle from the medial side through the vascular fissure. This fissure is located between the upper (dorsal) surface of the thalamus and the lower surface of the fornix.

Between the upper and lower layers of the vascular base of the third ventricle, in the connective tissue, are two internal cerebral veins (vv. cerebri internae). When these veins merge, they form an unpaired great cerebral vein (v. cerebri magna; Galen's vein). From the side of the ventricular cavity, the vascular base of the third ventricle is covered with an epithelial plate - a remnant of the posterior wall of the second brain vesicle. The outgrowths (villi) of the lower layer of the vascular base, together with the epithelial plate covering them, hang down into the cavity of the third ventricle, where they form a choroid plexus (plexus choroideus). In the area of the interventricular opening, this choroid plexus connects with the choroid plexus of the lateral ventricle.