Foramen magnum in Latin means great hole. It is the largest of several openings called foramen or canals in the base of the skull to allow for the passage of nerves, blood vessels, cerebrospinal fluid and connective tissue into and out of the brain. It is located in the anterior portion (front) of the occipital bone. The picture above shows the skull base looking down into it, the front of the skull is at the top and the foramen magnum is the largest opening and is not labeled.
There are several important structures that pass through the foramen magnum (FM). The medulla oblongata which is the upper portion of the spinal cord along with its membranes, the spinal accessory cranial nerve, the vertebral arteries, the anterior and posterior spinal arteries, and the accessory emissary venous drainage routes of the brain used for upright posture. In addition to the FM, the accessory drainage system of the brain also uses the hypoglossal and condylar canals which are closely connected to the foramen magnum due to their passage through the occipital condyles.
The occipital bone forms a large and important part of the base of the skull. It allows for the passage of many structures of the brain. It connects the skull to the upper cervical spine via condyles on it’s exterior base surface. Moreover, parts of the upper cervical spine develop from the same primitive tissues in the embryo as the base of the skull.
The occipital condyles are knuckle-like projections that drop down on either side of the FM and connect the skull to the upper cervical spine. The condyles are often pierced by tunnels that pass through them called the hypoglossal and condylar canals mentioned above.
Although, technically speaking, they are anatomically distinct from the foramen magnum, from a functional structural point of view, the inner walls of the condyles form part of the restricted confines of the foramen magnum. In other words, looking down through the foramen magnum from the top side, the condyles increase the depth of the front part of the foramen compared to the rear part. They also intrude slightly into the FM making the tunnel slightly smaller and more restricted for a considerable distance compared to the rear of the foramen. This makes the front of the foramen magnum more like a partial canal. The canals of the skull, like the optic canal and auditory meatus are naturally more restrictive to increases in volume of their contents such as swelling.
Looking at the picture below the thick stem-like projection of the occipital bone that lies anterior to (in front of) the foramen magnum is called the basilar portion. The curved surface of the occipital bone located at the rear of the foramen is called the squama and forms the rear wall of the cranial vault. On the inside of the cranial vault the major drainage routes of the brain called the transverse sinuses are located close to the sides of the occipital bone.
In additon to the link between the condyles of the occipital bone to the articular surfaces of the first cervical vertebra (C1) called atlas, the foramen magnum is connected to the upper cervical spine by connective tissues called the membrana tectoria and the alar ligaments which will be discussed further below.