These ventricles are filled with cerebrospinal fluid (CSF), which acts as a cushion and shock absorber for the brain, provides nutrients and removes waste products.

There are four main ventricles:

* Lateral ventricles (2): These are the largest ventricles and are located in each cerebral hemisphere.

* Third ventricle: This ventricle is located in the center of the brain, between the two thalami.

* Fourth ventricle: This ventricle is located in the brainstem, just below the third ventricle.

It's important to note that while the ventricles are hollow, they are not empty. They are filled with CSF which plays a vital role in brain health.