The septum pellucidum is the structure which lies at the medial wall of the lateral ventricle, and functions to physically separate the lateral ventricle from the third ventricle.
                                                            
                                                     
                        
                                                
                        
                        
                            A thin wall of brain tissue which divides the ventricles.
                                                            
                                                     
                        
                                                
                        
                        
                            The septum pellucidum (also called the septum lucidum) is a thin, triangular, vertical membrane that separates the lateral ventricles of the brain.