Foramen Magnum

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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
Latin	Foramen magnum
MeSH	D005539
TA	A02.1.04.002
FMA	75306
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 border; 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
  • Apical ligament and tip of dens
  • Upper band of Cruciate ligament
  • Membrana tectoria
• 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)

See also

• Posterior cranial fossa

References

This article incorporates text in the public domain from page 129 ofthe 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. CiteSeerX10.1.1.591.2458. doi:10.1016/j.jhevol.2013.07.007. PMID24055116.
2. Dutta, Asim Kumar (2013). Essentials of Human Anatomy Head & Neck. kolkata: Current books international. pp.56–57. ISBN978-81-86793-79-4.
3. Russo, Gabrielle A.; Kirk, E. Christopher (2013). "Foramen magnum position in bipedal mammals". Journal of Human Evolution. 65 (5): 656–70. CiteSeerX10.1.1.591.2458. doi:10.1016/j.jhevol.2013.07.007. PMID24055116. Lay summary – Phys.org (September 27, 2013).

External links

Wikimedia Commons has media related to Foramen magnum.

• "Anatomy diagram: 34257.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01.
• Diagram 1
• Diagram 2
• 3D animation showing position of basion on YouTube

Neurocranium of the skull
Occipital	Squamous part external Inion/External occipital protuberance Occipital bun External occipital crest Nuchal lines Suprainiac fossa planes Occipital Nuchal internal Cruciform eminence Internal occipital protuberance Internal occipital crest Groove for transverse sinus Lateral parts Condyle Condyloid fossa Condylar canal Hypoglossal canal jugular Jugular process Jugular tubercle Basilar part Pharyngeal tubercle Clivus Other Foramen magnum Basion Opisthion
Parietal	Parietal eminence Temporal line Parietal foramen Sagittal sulcus Sagittal keel Sagittal crest
Frontal	Squamous part Frontal suture Frontal eminence external Superciliary arches Glabella foramina Supraorbital foramen Brow ridge Foramen cecum Zygomatic process internal Sagittal sulcus Frontal crest Orbital part Ethmoidal notch Fossa for lacrimal gland Trochlear fovea Frontal sinus Frontonasal duct
Temporal	Squamous part Articular tubercle Suprameatal triangle Mandibular fossa Petrotympanic fissure Zygomatic process Mastoid part Mastoid foramen Mastoid process (Mastoid cells) Mastoid notch Occipital groove Sigmoid sulcus Mastoid antrum (Aditus) Petrous part Carotid canal Facial canal Hiatus Internal auditory meatus Cochlear aqueduct Stylomastoid foramen fossae Subarcuate fossa Jugular fossa canaliculi Inferior tympanic Mastoid Styloid process Petrosquamous suture (note: ossicles in petrous part, but not part of temporal bone) Tympanic part Suprameatal spine
Sphenoid	Surfaces Superior surface: Sella turcica Dorsum sellae Tuberculum sellae Hypophysial fossa Posterior clinoid processes Ethmoidal spine Chiasmatic groove Middle clinoid process Petrosal process Clivus Lateral surface: Carotid groove Sphenoidal lingula Anterior surface: Sphenoidal sinuses Great wings foramina Rotundum Ovale Vesalii Spinosum Spine Infratemporal crest Sulcus of auditory tube Small wings Superior orbital fissure Anterior clinoid process Optic canal Pterygoidprocesses fossae Pterygoid Scaphoid pterygoid plates Lateral Medial Pterygoid canal Hamulus Other Body Sphenoidal conchae
Ethmoid	Plates Cribriform plate Crista galli Olfactory foramina Perpendicular plate Surfaces Lateral surface Orbital lamina Uncinate process Medial surface Supreme nasal concha Superior nasal concha Superior meatus Middle nasal concha Middle meatus Labyrinth Ethmoid sinus ethmoidal foramina Posterior Anterior

Foramina of the skull (and canals, fissures, meatus, and hiatus)
Anterior cranial fossa	to Orbit: ethmoidal foramina anterior posterior to Nasal cavity: olfactory foramina (CN-I) foramen cecum
Middle cranial fossa	to Orbit: optic canal (CN-II) superior orbital fissure (CN-III,IV,V1,VI) to Pterygopalatine fossa: foramen rotundum (CN-V2) pterygoid canal to Infratemporal fossa: foramen ovale (CN-V3) foramen spinosum/carotid canal other: foramen lacerum hiatus for greater petrosal nerve hiatus for lesser petrosal nerve sphenoidal emissary foramen
Posterior cranial fossa	internal auditory meatus/facial canal/stylomastoid foramen (CN-VII,VIII) jugular foramen (CN-IX,X,XI) foramen magnum (CN-XI) hypoglossal canal (CN-XII) condylar canal mastoid foramen
Orbit	to Nasal cavity: nasolacrimal canal to face: supraorbital notch foramen infraorbital foramen groove zygomatic foramen -facial -temporal to Pterygopalatine fossa: inferior orbital fissure other: Inferior orbital fissure Fossa for lacrimal sac
Pterygopalatine fossa	to Nasal cavity: sphenopalatine foramen to Oral cavity: greater palatine canal foramen lesser palatine canals foramina to Infratemporal fossa: pterygomaxillary fissure to Nasopharynx: palatovaginal canal
to oral cavity: incisive canals incisive foramen to nasal cavity: foramen vomerinum meatus vomerinus fissura vomerina hiatus vomerinus
Other	external acoustic meatus mandibular foramen mental foramen parietal foramen petrotympanic fissure semilunar hiatus

Authority control	TA98: A02.1.04.002

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