The Base of the skull is formed by the fusion of Prechondral chondrocranium and chordal Chondrium which ossify by endochondral ossification. Foramen Magnum (FM) is a Latin word meaning the largest aperture in the base of the Skull. Foramen Magnum lies one-third in front and two-third behind the line formed by joining the tips of the mastoid process. It is oval and wider behind, with the greatest diameter (dm) being anteroposterior (AP). On each side, its anterolateral margin is encroached by Occipital Condyle (OC), hence the FM is narrow anteriorly and the posterior half of the Foramen Magnum is thin and semicircular. The Foramen Magnum is normally about 3 cm wide and 3.5 cm AP. The major vital neuroanatomic structures that pass through this largest foramen are medulla oblongata with the meninges, vertebral arteries, anterior spinal artery, posterior spinal arteries, and accessory spinal nerves. The apical ligament of dens and the tectorial membrane attach to the internal basiocciput.
