Why does the brain have ventricles?
To allow the brain to swell if it is injured. From an evolutionary point of view, the brain is the most important organ in the body. Accordingly, the skull evolved to provide strong protection from external injury. The skull does an excellent job, but there is one problem: it makes it difficult for the brain to swell outward.

Why would the brain need to swell?
Swelling is a fundamental aspect of inflammation, the process that occurs throughout the body in response to trauma or certain other types of injury. Swelling is associated with vascular dilation which allows the body to send extra oxygen, white blood cells and other microscopic particles to the site of injury, in order to fight injury or infection, and allow the organ to heal. The brain and spinal cord may be unique among organs in that they are encased in bony coverings, allowing them little room to swell.

If the brain was composed completely of parenchyma (tissue), it could expand only outward. But if it tried to expand outward, it would have almost no ability to do so, because it would be limited by the skull, which covers the brain fairly tightly. If it tried to expand outward anyway, it would injure itself, including vital regions (for example, those controlling breathing and heart rate) and cause death.

Evolution solved this problem by creating ventricles inside the brain. Ventricles are cavities normally filled with cerbrospinal fluid. After an injury, if the brain parenchyma swells, the ventricles will shrink to make room for the expanded parenchyma. The fluid that was in the ventricles gets drained or pushed out of the brain, into the venous system and eventually excreted from the body.

However, the ventricles can only shrink so much. If the brain swelling is so severe that the ventricles have shrunk basically completely, further brain swelling usually results in death. In order to prevent such a bad outcome in situations like these, neurosurgeons have perfected the technique of removing portions of the skull. This approach allows the brain to expand outward, often saving the patient’s life. After such a procedure, the skull fragment removed is stored in a freezer. Patients typically will wear a special tent or cover over the area of skull which has been removed, and wear a helmet to protect the head and brain. After the swelling subsides, the skull fragment is removed from the freezer, thawed, and reinserted in the patient’s skull.

Click here to see CNN’s article on CTE findings in deceased NFL football players.