Foramen magnum

The foramen magnum is a large, oval-shaped opening (foramen) in the occipital bone of the skull.[2] It is present in humans, and in many other animals. Anteriorly, it is bounded by the basiocciput.[2] Posteriorly, it is bounded by the supraocciput.[2] Laterally, it is bounded by the occipital condyles.[2]

On the occipital bone, the foramen magnum presents two midline cephalometric landmarks. The opisthion is the midpoint on the posterior margin of the foramen magnum. The basion is located at the midpoint on the anterior margin of the foramen magnum.

The alar ligament, which is attached on each side to the tubercle of occipital condyle, divides the foramen magnum into an anterior smaller compartment and a posterior larger compartment.[3]

From the size, position and where it is located the foramen magnum can help determine many factors. In humans, the foramen magnum is found to be located and positioned anteriorly. [4] In some rodents and mammals, the foramen magnum can be found anteriorly in the cranium. [4]

The foramen magnum varies in size between individuals. Earlier ossification of the occipital bone leads to a smaller foramen.[5]

The foramen magnum varies in size and shape when comparing different populations to each other. In humans, men tend to have a larger sized foramen magnum than women, but the overall shape is consistent.[6]

In humans, the foramen magnum is farther underneath the head than in the other great apes. Thus, in humans, the neck muscles (including the occipitofrontalis muscle) do not need to be as robust in order to hold the head upright. Comparisons of the position of the foramen magnum in early hominid species are useful to determine how comfortable a particular species was when walking on two limbs (bipedalism) rather than four (quadrupedalism).

The foramen magnum transmits a number of important structures between the neck and the neurocranium.

Structures passing through anterior compartment (osseoligamentous compartment) include:

• the apical ligament and tip of the dens.
• the upper band of cruciate ligament of the atlas (C1 vertebra).
• the membrana tectoria.

Structures passing through posterior compartment (neurovascular compartment) include:

• the lower end of medulla oblongata, surrounded by meninges.
• the fourth part of the vertebral artery, surrounded by sympathetic plexus of nerves.
• accessory nerves.
• anterior and posterior spinal arteries.
• the tonsil of the cerebellum (occasionally) as in a tonsillar herniation known as a Chiari malformation.

The foramen magnum may be too large, too small, or the wrong shape.[2] A small foramen magnum can cause neurological problems, and the reduced circulation of cerebrospinal fluid can cause hydrocephalus.[2] This may be treated with suboccipital craniectomy.[2]

An anterior foramen magnum can be found in other bipedal mammals besides humans.[8] The jerboa, a bipedal rodent, also has a foramen magnum.[8]

Although not fully proven, there are many studies that show the possibility[9] that where the foramen magnum is positioned in the cranium is significant in fossils. By being able to locate where the foramen magnum is positioned, anthropologists and other researchers are able to determine whether or not the species were bipedal (among other factors). The positioning of the foramen magnum changing over time can be seen in different fossils.[10] Analyzing the foramen magnum in fossils like Ardipithecus ramidus, links the fossil to other earlier and later hominid who share similar anterior foramen magnum and other bipedalism features. Although not proven, there are correlations with the foramen magnum being more anterior positioned to the ability of mammals to walk on two limbs. [9] Comparing this to other animals, such as some primates, the foramen magnum are located more of a posterior position in the cranium.[10] With the foramen magnum being position anterior in the cranium, the body of bipedal mammals is given a different center of gravity compared to quadrupedal mammals. The anterior foramen magnum shifts the weight of the body more to the mammals' pelvis and femur, present in some primates, like great apes. With a posterior foramen magnum, the alignment and weight of the body falls more lateral under the head This allows for humans and other bipedal mammals to be able to walk on two limbs. Even with a posterior foramen magnum, some primates are still able to use their lower two limbs to walk.

With a common anterior foramen magnum in fossils, the fossils can be studied and researched to link mammals to others who are potentially bipedal.[10] With more correlations and similarities of fossils who have an anterior foramen magnum; it links to the idea that these species might be bipedal. The shift in the foramen magnum moving towards a more anterior position in the cranium, gives rise to the idea of possible bipedalism.

• 
  Skull seen from below. The hole through which the medulla (shown in red) is passing is foramen magnum.
• 
  Opisthion shown in red
• 
  Occipital bone inner surface (basion shown in red)

• Posterior cranial fossa

This article incorporates text in the public domain from page 129 of the 20th edition of Gray's Anatomy (1918)

Wikimedia Commons has media related to Foramen magnum.

• "Anatomy diagram: 34257.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2012-07-22.
• Diagram 1 Archived 2016-03-04 at the Wayback Machine
• Diagram 2 Archived 2016-03-04 at the Wayback Machine
• 3D animation showing position of basion on YouTube