The third ventricle is one of the four connected fluid-filled cavities within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid (CSF). Please help to figure out license type for the image Comparative brain sizes. For other articles about other subjects named brain see brain (disambiguation). ... Cerebrospinal fluid (CSF) is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). ...

It is in the midline, between the left and right lateral ventricles. It communicates with the lateral ventricles anteriorly (in front), and with the mesencephalic duct (aqueduct of Sylvius) posteriorly (in back). The ventricular system is a fluid conducting system within the brain. ... The mesencephalic duct, also known as the aqueduct of Silvius or the cerebral aqueduct, contains cerebrospinal fluid (CSF), is within the mesencephalon (or midbrain) and connects the third ventricle in the thalamus (or diencephalon) to the fourth ventricle, which is between the pons and cerebellum. ...

Developmentally it represents the cavity of the diencephalon, though they are in front of the interventricular foramen which is derived from the median part of the telencephalon. The third ventricle is bounded by the thalamus and hypothalamus on both the left and right sides. The lamina terminalis forms the anterior wall of the third ventricle. The thalamus is a part of the brain. ... In the anatomy of mammals, the hypothalamus is a region of the brain located below the thalamus, forming the major portion of the ventral region of the diencephalon and functioning to regulate certain metabolic processes and other autonomic activities. ...

There are two protrusions on the front of the third ventricle, the supra-optic recess (above the optic chiasma), and the infundibular recess (above the pituitary stalk). In casts of the ventricular system, a small 'hole' may be seen in the body of the third ventricle. This is formed where the two thalami are joined together at the interthalamic adhesion (not seen in all people). The optic chiasm is the part of the brain where the optic nerves partially cross, those parts of the right eye which see things on the right side being connected to the left side of the brain, and vice versa. ... Located at the base of the skull, the pituitary gland is protected by a bony structure called the sella turcica. ...

The four fluid-filled cavities in the brain, collectively the ventricular system, are the left and right lateral ventricles, the third ventricle, and the fourth ventricle. The ventricular system is a fluid conducting system within the brain. ... The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. ...