Overview. The ventricles of the brain are a communicating network of cavities filled with cerebrospinal fluid (CSF) and located within the brain parenchyma. The ventricular system is composed of 2 lateral ventricles, the third ventricle, the cerebral aqueduct, and the fourth ventricle (see the following images).he largest cavities of the ventricular system are the lateral ventricles. Each lateral ventricle is divided into a central portion, formed by the body and atrium (or trigone), and 3 lateral extensions or horns of the ventricles. The central portion or the body of the ventricle is located within the parietal lobe.