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.

Upright Posture and the Foramen Magnum

The position of the foramen magnum in humans is unique compared to other mammals. In humans it has migrated well forward in the occipital bone from the rear of the skull, to a position beneath the center mass of the skull and brain. Compared to quadriped mammals and great apes for example, this creates better balance and so the neck bones and muscles are proportionately smaller as seen in the picture below.

On the other hand, the head and brain in humans are relatively larger compared to the small size of the upper cervical spine. Consequently, humans are especially susceptible to whiplash type injuries. Walking on two legs (bipedalism) is tricky and makes us susceptible to simple slips and falls. What’s more, when they do occur the relatively higher height of the head above the ground results in greater distance and time to hit the ground. In fact, slips and falls can further accelerate the fall by increasing the speed at which the feet slip foward, which whips the body backwards before it hits the ground. The increase in distance and time and the added speed of slipping feet increases the force and impact of even minor falls.

In addition to the impact of the relatively unstable and high position of the head above the feet due to upright posture, the relatively small muscles of the neck due to upright posture provide far less protection compared to other primates or mammals. Over the course of a lifetime, whiplash type trauma from simple slips and falls, never mind motor vehicle accidents, can be cumulative, that is, they add up and eventually take their toll on the upright spine, brain and cord.

Dura Mater and Dentate Ligament Connections

The membrana tectoria is also known as the tectorial membrane of the occipitoaxial ligaments. The tectorial membrane of the occipitoaxial ligaments is a stong broad band of connective tissue that covers the odontoid process of the axis vertebra (second cervical vertebra – C2) of the upper cervical spine and its ligaments. The odontoid process is the tall thumb-like projection of the second cervical vertebra that sticks upwards in front of the spinal canal beneath the skull in the picture below. The odontoid process is also sometimes called the dens because it looks like a tooth.

The top picture is a side view of the occiput, FM, upper cervical vertebra, tectorial membrane of the occipitoaxial ligaments in the upper cervical spine, as well as the posterior longitudinal ligaments of the rest of the spine. The second picture is looking at this area from the back side of the head. In this picture the tectorial membrane of the occipitoaxial ligaments has been cut and folded down to show the other upper cervical ligments that lie in front of it.

Some anatomists consider the membrana tectoria to be a continuation of the posterior longitudinal ligament of the vertebral column seen in the picture below. It is fixed below to the posterior surface of the body of the axis (C2). It gets wider as it rises upwards and attaches to the basilar groove of the occipital bone in front of the foramen magnum. At the level of the FM, the anterior (front) surface of the tectorial membrane blends with the transverse ligament of the atlas vertebra (C1). The posterior (rear) surface blends into the cranial dura mater of the brain.

The dura mater is the outer of the three protective coats that surround the brain and cord. Craniopathic practioners in chiropractic and osteopathy have long maintained that misalignments of the spine cause dural tension in the brain and cord. The reason for this is the connection of the dura mater of the brain to the foramen magnum, membrane tectoria and the upper cervical ligaments and muscles. At the caudal (bottom) end of the spine, the dura mater attaches to the segments of the spinal canal inside the sacrum of the pelvis.

In addition to the dura mater, the pia mater of the cord also appears to attach to the foramen magnum. The pia mater is the inner of the three layers of protective coats that surround the brain and cord. The dentate ligament is part of the pia mater.

Among other things, Dr. John D. Grostic, the research director at Life Chiropractic College, published a paper in 1988 in the Chiropractic Research Journal called The Dentate Ligament-Cord Distortion Hypothesis. The paper was based on research he did at Palmer Chiropractic College that showed that the dentate ligament of the cord attaches to the foramen magnum. The point of his theory was to attempt to explain some of the unusual signs and symptoms associated with upper cervical subluxations. (Dr. John D. Grostic is the son of John F. Grostic, the founder of the Grostic Upper Cervical method. The Grostic method formed the foundation of NUCCA upper cervical developed by Dr. Ralph Gregory).

Misalignments and curvatures of the spine are transmitted throughout the entire spinal column, skull, brain and cord via the dura mater and dentate ligament connections to the foramem magnum, upper cervical spine, spinal canal and tailbone. Furthermore, dural and dentate ligament tension can result in deformation and diplacement of the brain and cord in the cranial vault and spinal canal. Among other things, displacement of the brain within the cranial vault can cause cerebellar tonsillar ectopia. Cerebellar tonsillar ectopia, in turn, causes compression of the brainstem, cerebellum, blood and cerebrospinal fluid pathways in the foramen magnum and upper cervical canal, as well as against other parts of the base of the cranial vault.

Tethered Cord

In addition to upper cervical misalignments, spondylosis (degeneration), scoliosis (abnormal sidewards curve), kyphosis (abnormal backwards curve) and stenosis (narrowing of the spinal canal) can cause dural and dentate ligament tension. Likewise, a tethered cord can stress and strain the brain causing dural and dentate ligament tension.

On the other hand, spondylosis (degeneration of the spine) and scar tissue from injuries can tether the cord at the involved segments of the spine. Scoliosis and kyphosis (abnormal curves) can also cause tethered cords due to the exaggerated curves stretching and straining the cord in abnormal directions. Lastly, some people are born with naturally short cords that cause tethering at the tail end attachment of the cord, called the filum terminale, to the tailbone called the coccyx.

Because of dura mater, dentate ligament (pia mater) and filum terminale (pia mater) connections, spondylosis, scoliosis, kyphosis and tethered cords can restrain, traction and displace the contents of the cranial vault. Traction pulls the brainstem downwards toward the foramen magnum. Tonsillar ectopia occurs when the brainstem or cerebellum gets pushed from above, such as from over crowding in the posterior fossa, or it gets pulled down from below. In either case the cerebellum gets displaced into the foramen magnum.

In addition to the tonsils of the cerebellum, tonsillar ectopia also compresses the other contents of the foramen magnum mentioned above including nerves, venous blood vessels and CSF pathways. Compression of blood vessels can lead to decreased blood flow and drainage of the brain. Compression of CSF pathways can lead to hydrocephalus.

Consequently, among other things, the foramen magnum may play a prominent role in the cause of many neurodegenerative diseases such as Alzheimer’s, Parkinson’s and multiple sclerosis to name a few. It is time to stop looking at these structures as separate and distinct, but as a closely connected and highly interdependent system.