Inside the brain, there are four cavities, called ventricles. The right and left lateral ventricles and the third and fourth ventricles compose the ventricular system.

The fourth ventricle contains cerebrospinal fluid. It has a diamond shape and is located in the upper portion of the medulla. Specifically, it spans from the obex — an area in the medulla oblongata, located in the brainstem — to the cerebral aqueduct — a canal-like structure in the upper part of the brainstem that connects the fourth ventricle to the third. The main function of this ventricle is to protect the human brain from trauma (via a cushioning effect) and to help form the central canal, which runs the length of the spinal cord.

This ventricle has a roof and a floor. The roof is composed of the cerebellum, located at the back of the brain, and the floor is formed by the rhomboid fossa, a depression in the brainstem. Within the floor is the facial colliculus, sulcus limitans, and the obex.

Much like other portions of the ventricular system inside the brain, the fourth ventricle develops from the area inside the neural tube (the structure in an embryo that becomes the brain and spinal cord) called the central canal. During gestation, the ventricles form during the first three months of pregnancy. During this time of development, it is possible for circulation to be blocked by overproduction of cerebrospinal fluid, causing a condition called hydrocephalus.