Third Ventricle

The third ventricle is a narrow, four-sided, irregularly shaped opening in the middle of the brain that provides a pathway for cerebrospinal fluid. Cerebrospinal fluid is the cushiony fluid that protects the brain and spine from trauma. In addition to the third ventricle, there is a fourth ventricle and two lateral (side) ventricles, pictured below. There is one lateral ventricle on each side of the brain. Cerebrospinal fluid is produced by a cluster of blood vessels that line the ventricles. These clusters of blood vessels are known as the choroid plexus. Together, the ventricles of the brain hold about a glassful of cerebrospinal fluid.

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As you can see from the picture above, the third ventricle is located below and between the two lateral ventricles. The third ventricle has a narrow roof, a floor, and four walls. The wall in the back is known as the posterior wall. The wall in the front is known as the anterior wall. The walls on the side are known as the lateral walls.
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On each side of the third ventricle is a thalamus and the hypothalamus. The thalamus is a pair of large oval structures that sends out messages regarding sensation. The hypothalamus is an area below the thalamus that is important for many bodily functions such as sleep, appetite, and temperature control. For those who want more specific detail, the structures that surround the third ventricle are described below and divided by area:

FLOOR: The floor of the third ventricle is formed by the following structures: the optic chiasm, mammillary bodies, tuber cinereum, hypothalamus, subthalamus, infundibulum, posterior perforated substance, and the upper part of the midbrain tegmentum. A description of these terms follows. The optic chiasm is an area in the brain where optic nerves (nerves that help us see) cross over. The mammillary bodies are small, round, paired group of cells located in the bottom part of the hypothalamus. You can see where the third ventricle, other ventricles, optic nerves, and hypothalamus are located in the picture below to the right.

The tuber cinereum is a bulge at the bottom of the hypothalamus (see above). The subthalamus is an area of the brain located between the hypothalamus and the midbrain tegmentum (see below for description) that processes information about vision and balance.

The infundibulum is a funnel-shaped stalk that extends from the hypothalamus to the back part of the pituitary gland. The pituitary gland is an area in the brain that secretes (forms and gives off) chemicals known as hormones, such as those that are important for growth. Hormones are natural chemicals produced by the body and released into the blood that have a specific effect on tissues in the body.

The posterior perforated substance is the bottom part of the interpeduncular fossa that allows plenty of room for branches of arteries (types of blood vessels that carry blood away from the heart) to pass through. The interpeduncular fossa is a deeply sunken in area of the midbrain. The midbrain is the top part of the brainstem. The brainstem is an area in the lower part of the brain that connects it with the spinal cord. The midbrain tegmentum is a term for the major part of the midbrain. The midbrain tegmentum is located in the top part of the midbrain.

Below is a picture of the midbrain and the rest of the brainstem (pons and medulla). You can see how the top of the midbrain forms part of the floor of the third ventricle. The third ventricle is located directly above the midbrain.

ROOF: The narrow roof of the third ventricle is formed by a thin membrane known as the tela choroidea. The tela choroidea is a term for the combination of pia mater and ependyma, which are both types of membranes. A membrane is a thin layer of flexible tissue that covers something. Whereas ependyma is a type of membrane that lines the ventricles of the brain, pia mater is a type of membrane that lines other brain areas as well. The tela choroidea are attached on each side to the tenia thalami. The tenia thalami is the sharp edge or angle between the top and middle surfaces of the thalamus (see picture).


Recesses are small hollow areas or indentations. There are several recesses in the third ventricle:

PREOPTIC RECESS: This is located in the sharp angle between the bottom of the lamina terminalis and the back of the optic chiasm. The lamina terminalis is a thin plate that passes upwards from the optic chiasm (see picture). The optic chiasm is an area in the brain where optic nerves (nerves that help us see) cross over.

INFUNDIBULAR RECESS: This funnel shaped area extends from the front part of the third ventricle into the infundibulum (see previous section for description).

MAMMILLARY RECESS: This indentation is caused by the mammillary bodies (see picture) sticking into the third ventricle. The mammillary recess is also known as the inframammillary recess.

PINEAL RECESS: From the back corner of the third ventricle, this recess extends into the stalk of the pineal gland. See the last section for a description and picture of the pineal gland.

The third ventricle is connected to each of the lateral ventricles and to the fourth ventricle. The area that connects the third ventricle with each of the lateral ventricles is known as the interventricular foramen of Monro (also known as the interventricular foramen and the foramen of Monro). The area that connects the third ventricle with the fourth ventricle is known as the cerebral aqueduct (also known as the sylvian aqueduct). The third ventricle extends from the lamina terminalis (see above) to the head of the cerebral aqueduct.


The lateral ventricle is also known as the ventriculus tertius and the ventricle of diencephalon.


Third ventricle comes from the Greek word "triotus" meaning "below second rank" and the Latin word "venter" meaning "belly." Put the words together and you have "belly below second rank."