Cavities within the brain are termed

There are four cavities within the brain that are called cerebral ventricles. 

The two lateral ventricles (paracoels or I and II ventricles) are located in the two cerebral hemispheres. The third ventricle (diocoel) of diencephalon communicates with the lateral ventricles through two interventricular foramina, or foramina of Monro. From the third ventricle a narrow channel called the cerebral aqueduct, iter or aqueduct of Sylvius runs into the fourth ventricle. The fourth ventricle (myelocoel) is in the medulla.