<prompt> Floor of 4th ventricle is not formed by ?Sulcus limitansAnterior medullary velumPosterior surface of ponsPosterior surface of medulla  <response>: Floor of the fouh ventricle is not formed by Anterior medullary velum.Anterior medullary velum forms the roof.All the other options are associated with the floor of the ventricle. Fouh ventricle: It is a tent like structure, it is situated in front of the cerebellum and behind the pons and upper half of the medulla oblongata. It is lined by ciliated epithelium. Communications: 1. Cerebral aqueduct of Sylvius: in the superior median plane superior angle, it communicates with third ventricle. 2. Central spinal canal: in the inferior median plane inferior angle. Boundries: 1. Roof consists of Upper poion of roof: The superior peduncle and the anterior medullary velum; Lower poion of roof: The posterior medullary velum, the epithelial lining of the ventricle covered by the tela chorioidea inferior and the taeniae of the fouh ventricle. 2. Floor: Upper pa :Fascial colliculus on the dorsal surface of pons. Intermediate pa:Vestibular nuclei medulllary striae. Lower pa:Upper pa of medulla oblongata containing hypoglossal and vagal triangles. 3. Laterally: bounded by superior and inferior cerebellar peduncles,gracile tubercle and cuneate tubercle. Apeures: Through these, the CSF leaves the fouh ventricle to the subarachnoid space One median foramen foramen of Magendie: in the lower pa of the inferior medullary velum. Two lateral foramens foramens of Lushka: one in each lateral recess. Ref: 1. Color atlas of neuroscience: neuroanatomy and neurophysiology, By Ben Greenstein, Adam. Greenstein, Page 16 2. BD Chaurasias Human Anatomy,Volu III, 4Th edition, Page 397 