Median aperture
The median aperture (also known as the medial aperture, and foramen of Magendie) is an opening of the fourth ventricle at the caudal portion of the roof of the fourth ventricle. It allows flow of cerebrospinal fluid (CSF) from the fourth ventricle into the cisterna magna. The other two openings of the fourth ventricle are the lateral apertures - one on either side. Nonetheless, the median aperture accounts for most of the outflow of CSF out of the fourth ventricle. The median aperture varies in size. Anatomy

Relations 
The median foramen on axial images is posterior to the pons and anterior to the caudal cerebellum. It is surrounded by the obex and gracile tubercles of the medulla, tela choroidea of the fourth ventricle and its choroid plexus, which is attached to the cerebellar vermis. Eponym
The foramen of Magendie is named for François Magendie, who first described it. Additional images

References

Ventricular system