Within the brain lie a series of cavities that are responsible for the production of cerebrospinal fluid (CSF). These four cavities constitute the ventricular system: the first and second ventricles, referred to as the lateral ventricles due to their position in each cerebral hemisphere; the third ventricle, a medially situated cavity lying between the cerebral hemispheres; and the fourth ventricle which lies at the level of the brainstem. The third ventricle receives CSF from the lateral ventricles and conveys it to the fourth ventricle, which disseminates it to the subarachnoid space. The third ventricle therefore serves as the intermediary between the lateral ventricles and the fourth ventricle. While subsequent ventricles receive CSF from those upstream, each ventricle also contributes to the production of CSF. CSF has a nutritional and protective function to the brain and spinal cord. It is also important to note that the entire ventricular system is lined by an epithelial covering called the ependyma, which is functionally important in the production of CSF, and the maintenance of the blood-CSF barrier. In terms of location, the third ventricle is specifically located in the diencephalon and it is thus referred to as the cavity of the diencephalon. It is a median fossa situated between the right and left thalami.
The third ventricle is bounded laterally by the thalamus (superiorly), the hypothalamus (anteroinferiorly), and the subthalamus (posteroinferiorly). It communicates superolaterally with the first and second ventricles through the interventricular foramen (of Monro). Posteroinferiorly, it connects with the fourth ventricle through the cerebral aqueduct (of Sylvius). Like other ventricles, the third ventricle has a cavity, an anterior wall, a posterior wall, a floor, a roof and two lateral walls.
The floor & roof
The floor is formed by the optic chiasma, the tuber cinereum and the infundibulum, the mamillary bodies, the posterior perforated substance and the tegmentum of the midbrain. The roof is constituted by ependyma that stretches across the two thalami and lines the ventricular surface. Above this ependyma, there is a tela choroidea (a membrane consisting of ependyma and a double fold of pia mater). Within the tela choroidea are two plexuses of blood vessels that bulge downwards into the cavity of the third ventricle; these are the choroid plexuses of the third ventricle, which are also very important in the formation of CSF.
The cavity of the third ventricle is marked by a number of elongations and recesses (extensions). All of its recesses are named according to their related structures; notably, the optic and infundibular recess. The infundibular recess extends into the infundibulum, and the optic recess is located above the optic chiasma. Also of note is the suprapineal recess which lies above the pineal body, and the pineal recess which lies between the superior and inferior laminae of the stalk of the pineal body.
The walls of the third ventricle have some striking features that are formed by their associated structures. The superior parts of the third ventricle are formed by the anterior commissure as well as by the columns of the fornix as they diverge from each other. The inferior portion of the anterior wall is formed by the lamina terminalis, which bridges between the optic chiasm and the rostrum of the corpus callosum. The pineal body and the posterior commissure lie posterior to the ventricle forming its posterior wall.
Each of the two lateral walls of the third ventricle is marked by the hypothalamic sulcus, which follows the curved course from the interventricular foramen to the aqueduct. The lateral walls below the hypothalamic sulcus are formed by the medial surface of the hypothalamus anteriorly and the subthalamus posteriorly. The left and right thalami are also major structures forming the lateral walls of the third ventricle. The thalamus lies just above the hypothalamic sulcus on the medial surface of the cerebrum. The thalami form a major part of the lateral walls of the third ventricle, and both thalami connect to form the roof of the third ventricle via a band of gray matter known as the interthalamic connexus. The epithalamus also forms a small portion of the lateral wall, and lies above and behind the thalamus. The interventricular foramen, through which the third ventricle communicates with the lateral ventricles, is seen on the lateral wall just behind the column of the fornix.
Choroid plexuses and Tela choroidea
The tela choroidea of the third ventricle originates from branches of the internal carotid artery and gives rise to the choroid plexus. It is made up of a fold in the vascular pia mater that is in close proximity to the ependymal layer and occupies the interval between the splenium of the corpus callosum and the fornix. The choroid plexus is a complex of vessels, bulging into the ventricular space, that actively secrete plasma, electrolytes, nutrients, and small proteins through fenestrations. The third ventricle has two choroid plexuses running parallel to each other, one on either side of the mid line. These choroid plexuses and the tela choroidea function to produce the cerebrospinal fluid (CSF), which fills the subarachnoid space and ventricular system.