What is the Third Ventricle?

The third ventricle is one of four fluid-filled cavities in the ventricular system within the human brain. It extends from the central canal of the spinal cord and is filled with cerebrospinal fluid (CSF). The other four ventricles consist of two lateral ventricles and the fourth ventricle. Together, they provide protection and stability for the brain. This makes the ventricular system essential for proper neural development and growth in humans.

The third ventricle develops in the same way as the rest of the ventricular system — from the central canal of the neural tube. More specifically, this ventricle comes from the part of the neural tube that is present in the forebrain. This is also known as the prosencephalon.

The third ventricle is a narrow cavity between the two lateral ventricles in the ventricular system. The third ventricle is essentially the midline cavity in the brain that lies on top of the optic chiasm, tuber cinereum, the mamilary bodies, the infundibulum, the posterior perforated substance, and the uppermost part of the mesencephalic tegmentum. It is also bound by the thalamus and hypothalamus on either side. The roof of the third ventricle consists of a layer of ependyma, which is a thin epithelial membrane lining.

The third ventricle also has several small hollow areas or indentations called recesses. The preoptic recess is at the back of the optic chiasm, which is where the optic nerves cross over into the brain. Next, the infundibular recess is a funnel-shaped area extending into the infundibulum. The mamilary bodies sticking into the cavity cause the third recess to be named the mamilary recess. Lastly, the pineal recess is in the posterior corner of the third ventricle and extends into the pineal gland.

Connections between the four ventricles allow for the flow of CSF between the third ventricle and the others. This ventricle, in particular, communicates with the lateral ventricles anteriorly via the interventricular foramen. It also communicates with the fourth ventricle posteriorly via the cerebral aqueduct, which is also known as the aqueduct of Sylvius.

The main purpose of the ventricular system is to produce and circulate the CSF around the brain. This cushions the brain and spinal cord from damage within the skull. A concussion, or bruising on the brain, results from contact with the skull during a hard jolt. The CSF also provides buoyancy to support the brain against gravity and provide for a floating brain.