Foramen magnum

The foramen magnum is a largest opening in the base of the skull, located within the occipital bone, inside the posterior cranial fossa. It is surrounded at the back by the squamous portion of the occipital bone, on the sides by the lateral condylar parts of the same bone, and in the front by the basilar part of the occipital bone. It’s front part is narrow due to being overlapped by the inner surfaces of the occipital condyles.

The edges of the foramen magnum serve as points of attachment for several ligaments at the skull’s base. For instance, its front and back edges attach to the anterior and posterior atlantooccipital membranes, respectively, while the alar ligaments attach to the inner sides of the occipital condyles.

The foramen magnum allows the passage of the medulla oblongata and its protective meningeal layers, including the spinal dura mater, arachnoid mater, and pia mater. It also allows the passage of various arteries such as the anterior and posterior spinal arteries, vertebral arteries, nerves like the spinal accessory nerves, and the sympathetic plexus around the vertebral arteries.

Furthermore, several ligaments run through the foramen magnum’s narrow front part and connect to the lower section of the basilar part of the occipital bone. These include the apical ligament of dens, the upper vertical band of the cruciate ligament, and the tectorial membrane.