has part has part has_part characteristic of part of has modifier abnormal brain ventricular system morphology abnormal median aperture morphology abnormal foramen of Magendie morphology MP:0009802 abnormal arachnoid foramen morhpology any structural anomaly in 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 morphology abnormal fourth ventricle median aperture