Foramen magnum

The foramen magnum (Latin: great hole) is a large oval opening (foramen) in the occipital bone of the skull in humans and many other animals. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblongata, passes through the foramen magnum as it exits the cranial cavity. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the anterior and posterior spinal arteries, the tectorial membranes and alar ligaments. It also transmits the accessory nerve into the skull.

Foramen magnum
Upper surface of base of the skull. The hole indicated by an arrow is the foramen magnum
Occipital bone. Inner surface.
Details
Identifiers
LatinForamen magnum
MeSHD005539
TAA02.1.04.002
FMA75306
Anatomical terms of bone

The foramen magnum is a very important feature in bipedal mammals. One of the attributes of a biped's foramen magnum is a forward shift of the anterior borderof the cerebellar tentorium; this is caused by the shortening of the cranial base. Studies on the foramen magnum position have shown a connection to the functional influences of both posture and locomotion. The forward shift of the foramen magnum is apparent in bipedal hominins→, including modern humans, Australopithecus africanus, and Paranthropus boisei. This common feature of bipedal hominins is the driving argument used by Michel Brunet that Sahelanthropus tchadensis was also bipedal, and may be the earliest known bipedal ape. The discovery of this feature has given scientists another form of identifying bipedal mammals. [1]

Landmarks

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.

Compartments

The alar ligament which is attached on each side to the tubercle of occipital condyle on each side of the foramen magnum divides it into an anterior smaller compartment and a posterior larger compartment.[2]

  • Structures passing through anterior compartment (osseo-ligamentous compartment) are
  • Structures passing through posterior compartment (Neuro-vascular compartment) are
    • Lower end of Medulla Oblongata with meninges
    • Fourth part of vertebral artery surrounded by sympathetic plexus of nerves
    • accessory nerves
    • anterior and posterior spinal arteries
    • tonsil of cerebellum (occasionally) as in a tonsillar herniation known as a Chiari malformation

Other animals

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 jerboa, a bipedal rodent, also has a foramen magnum.[3]

Additional images
  • 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)
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gollark: Sure I do. Your abstract thinking is just bad. Some offense.
gollark: Some systems will conveniently go back to an equilibrium regardless of how hard you poke them. Some will not, and might just vary wildly or get stuck in one state or whatever.
gollark: That isn't actually true except in specific technical contexts.
gollark: https://upload.wikimedia.org/wikipedia/commons/6/63/Logistic_Map_Animation.gif

See also

References

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

  1. Russo, Gabrielle A.; Kirk, Christopher E. (November 2013). "Foramen magnum position in bipedal mammals". Journal of Human Evolution. 65 (5): 656–670. CiteSeerX 10.1.1.591.2458. doi:10.1016/j.jhevol.2013.07.007. PMID 24055116.
  2. Dutta, Asim Kumar (2013). Essentials of Human Anatomy Head & Neck. kolkata: Current books international. pp. 56–57. ISBN 978-81-86793-79-4.
  3. Russo, Gabrielle A.; Kirk, E. Christopher (2013). "Foramen magnum position in bipedal mammals". Journal of Human Evolution. 65 (5): 656–70. CiteSeerX 10.1.1.591.2458. doi:10.1016/j.jhevol.2013.07.007. PMID 24055116. Lay summary – Phys.org (September 27, 2013).
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