has part has part has_part characteristic of has modifier abnormal median aperture morphology absent median aperture MP:0009805 absence of the large midline opening of the posterior inferior part of the roof of the fourth ventricle that connects the fourth ventricle to the posterior cerebromedullary cistern and the spinal cord MPheno.ontology absent foramen of Magendie absent arachnoid foramen abnormal absent fourth ventricle median aperture