View source for Foramen magnum

{{short description|Opening in the occipital bone of the skull}}
{{more footnotes|date=January 2013}}
{{Infobox bone
| Name        = Foramen magnum
| Latin       = foramen magnum
| Image       = Crane4 Foramen magnum.png
| Caption     = Upper surface of the [[base of skull]]; the hole indicated by an arrow is the foramen magnum.
| Image2      = Gray130.png
| Caption2    = Occipital bone. Inner surface.
|PartOf=[[Occipital bone]]|System=[[Skeletal system|Skeletal]]}}
The '''foramen magnum''' ({{Langnf|la||great hole}}) is a large, oval-shaped opening in the [[occipital bone]] of the [[skull]]. It is one of [[list of foramina of the human body|the several oval or circular openings (foramina)]] in the [[base of skull|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 artery|vertebral arteries]], the [[Anterior spinal artery|anterior]] and [[Posterior spinal artery|posterior spinal arteries]], the [[Tectorial membrane of atlanto-axial joint|tectorial membranes]] and [[alar ligament]]s. It also transmits the [[accessory nerve]] into the skull.

The foramen magnum is a very important feature in [[bipedalism|bipedal]] mammals. One of the attributes of a biped's foramen magnum is a forward shift of the anterior border of 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 [[hominini|hominins]], including modern humans, ''Australopithecus africanus'', and ''Paranthropus boisei''. This common feature of bipedal [[hominini|hominins]] is the driving argument used by [[Michel Brunet (paleontologist)|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.<ref>{{cite journal|last1=Russo|first1=Gabrielle A.|last2=Kirk|first2=Christopher E.|title=Foramen magnum position in bipedal mammals|journal=Journal of Human Evolution|date=November 2013|volume=65|issue=5|pages=656–670|doi=10.1016/j.jhevol.2013.07.007|pmid=24055116|citeseerx=10.1.1.591.2458}}</ref>

== Structure ==
The foramen magnum is a large, oval-shaped opening ([[foramen]]) in the occipital bone of the [[skull]].<ref name=":0">{{Cite book|last1=Sanchez|first1=Pedro|chapter-url=https://www.sciencedirect.com/science/article/pii/B978032337101800031X|title=Swaiman's Pediatric Neurology|last2=Graham|first2=John M.|publisher=[[Elsevier]]|year=2017|isbn=978-0-323-37101-8|edition=6th|pages=233–241|language=en|chapter=31 - Congenital Anomalies of the Skull|doi=10.1016/B978-0-323-37101-8.00031-X}}</ref> It is present in [[Human|humans]], and in many other [[Animal|animals]]. Anteriorly, it is bounded by the basiocciput.<ref name=":0" /> Posteriorly, it is bounded by the supraocciput.<ref name=":0" /> Laterally, it is bounded by the [[occipital condyles]].<ref name=":0" />

On the occipital bone, the foramen magnum presents two midline [[cephalometric analysis#Cephalometric landmarks|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.<ref name="A.K. DUTTA 2013">{{cite book|last=Dutta|first=Asim Kumar|title=Essentials of Human Anatomy Head & Neck|year=2013|publisher=Current books international|location=Kolkata|isbn=978-81-86793-79-4|pages=56–57}}</ref>

== Variation ==
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. <ref name=":1">{{Cite journal |last1=Russo |first1=Gabrielle A. |last2=Kirk |first2=E. Christopher |date=2017-04-01 |title=Another look at the foramen magnum in bipedal mammals |url=https://www.sciencedirect.com/science/article/pii/S0047248417300507 |journal=Journal of Human Evolution |language=en |volume=105 |pages=24–40 |doi=10.1016/j.jhevol.2017.01.018 |pmid=28366198 |issn=0047-2484|url-access=subscription }}</ref> In some rodents and mammals, the foramen magnum can be found anteriorly in the cranium. <ref name=":1" />

The foramen magnum varies in size between individuals. Earlier [[Endochondral ossification|ossification]] of the occipital bone leads to a smaller foramen.<ref name=":02">{{Cite book |last1=Sanchez |first1=Pedro |chapter-url=https://www.sciencedirect.com/science/article/pii/B978032337101800031X |title=Swaiman's Pediatric Neurology |last2=Graham |first2=John M. |publisher=[[Elsevier]] |year=2017 |isbn=978-0-323-37101-8 |edition=6th |pages=233–241 |language=en |chapter=31 - Congenital Anomalies of the Skull |doi=10.1016/B978-0-323-37101-8.00031-X}}</ref>

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.<ref>{{Cite journal |last1=Zdilla |first1=Matthew J |last2=Russell |first2=Michelle L |last3=Bliss |first3=Kaitlyn N |last4=Mangus |first4=Kelsey R |last5=Koons |first5=Aaron W |date=2017 |title=The size and shape of the foramen magnum in man |journal=Journal of Craniovertebral Junction & Spine |volume=8 |issue=3 |pages=205–221 |doi=10.4103/jcvjs.JCVJS_62_17 |issn=0974-8237 |pmc=5634107 |pmid=29021672 |doi-access=free }}</ref>

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]]).

=== Heart-shaped foramen magnum ===
In an early hominin, ''[[Paranthropus boisei]]'', also known as ''Australopithecus boisei'', has a foramen magnum that is similar to a shape of a heart or a [[cardioid]].<ref name=":22">{{Cite journal |last1=Tobias |first1=Phillip V. |last2=Symons |first2=Joel A. |date=1992 |title=Functional, Morphogenetic and Phylogenetic Significance of Conjunction between Cardioid Foramen Magnum and Enlarged Occipital and Marginal Venous Sinuses |url=https://www.jstor.org/stable/40386950 |journal=Archaeology in Oceania |volume=27 |issue=3 |pages=120–127 |doi=10.1002/j.1834-4453.1992.tb00295.x |jstor=40386950 |issn=0728-4896|url-access=subscription }}</ref>  When [[KNM-ER 406]] was examined among other species, the foramen magnum was different. Compared to the usual oval-shaped foramen magnum, the shape of the Australopithecus boisei foramen magnum appears to be shortened. In the foramen magnum of Australopithecus boisei, both the anterior and posterior are different in size and diameter. The anterior of the foramen magnum had a greater lateral diameter and was more broad compared to other early hominins. While the lateral diameter of the posterior foramen magnum was shortened to create more of a cardioid shape. The anterior edges of the foramen magnum in Australopithecus boisei lacks the oval curve that would normally appear in other species that have an oval to circular-shaped foramen magnum. The unique shape of the foramen magnum was also seen in some casts of Hadar species. <ref name=":22" /> The Hadar shows traits and characteristics of having a similar cardioid shape. <ref name=":22" />The change in the shape of the foramen magnum has been studied to identify the possibility of different functions. There is a potential correlation between the change of shape of the foramen magnum to how it functions in the body.  Although not proven, compared to other hominins, the route that blood travels through the foramen magnum in Australopithecus boisei are more straight and direct. <ref name=":22" /> With the enlargement and broadening of the anterior foramen magnum, allow for more venous drainage from the occipital and marginal sinuses. <ref name=":22" />

== Function ==
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 (anatomy)|dens]].
* the upper band of [[Cruciate ligament of atlas|cruciate ligament]] of the [[Atlas (anatomy)|atlas]] ([[Cervical vertebrae|C1 vertebra]]).
* the [[Tectorial membrane of atlanto-axial joint|membrana tectoria]].

Structures passing through posterior compartment (neurovascular compartment) include:
* the lower end of [[Medulla Oblongata|medulla oblongata]], surrounded by [[meninges]].
* the fourth part of the [[vertebral artery]], surrounded by sympathetic plexus of nerves.
* [[accessory nerve]]s.
* anterior and posterior [[Spinal arteries (disambiguation)|spinal arteries]].
* the tonsil of the [[cerebellum]] (occasionally) as in a tonsillar herniation known as a [[Chiari malformation]].

== Clinical significance ==
The foramen magnum may be too large, too small, or the wrong shape.<ref name=":0" /> A small foramen magnum can cause [[Neurological disorder|neurological problems]], and the reduced circulation of cerebrospinal fluid can cause [[hydrocephalus]].<ref name=":0" /> This may be treated with suboccipital [[Decompressive craniectomy|craniectomy]].<ref name=":0" />

== Other animals ==
An anterior foramen magnum can be found in other bipedal mammals besides humans.<ref name="Russo and Kirk22">{{Cite journal |last1=Russo |first1=Gabrielle A. |last2=Kirk |first2=E. Christopher |date=2013-11-01 |title=Foramen magnum position in bipedal mammals |url=https://www.sciencedirect.com/science/article/pii/S0047248413001681 |journal=Journal of Human Evolution |language=en |volume=65 |issue=5 |pages=656–670 |doi=10.1016/j.jhevol.2013.07.007 |pmid=24055116 |issn=0047-2484|url-access=subscription }}</ref> The [[jerboa]], a bipedal rodent, also has a foramen magnum.<ref name="Russo and Kirk22" />

== Evolution of foramen magnum in fossils ==
Although not fully proven, there are many studies that show the possibility<ref name="Russo and Kirk223">{{Cite journal |last1=Russo |first1=Gabrielle A. |last2=Kirk |first2=E. Christopher |date=2013-11-01 |title=Foramen magnum position in bipedal mammals |url=https://www.sciencedirect.com/science/article/pii/S0047248413001681 |journal=Journal of Human Evolution |language=en |volume=65 |issue=5 |pages=656–670 |doi=10.1016/j.jhevol.2013.07.007 |pmid=24055116 |issn=0047-2484|url-access=subscription }}</ref> 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.<ref name=":13">{{Cite journal |last1=Russo |first1=Gabrielle A. |last2=Kirk |first2=E. Christopher |date=2017-04-01 |title=Another look at the foramen magnum in bipedal mammals |url=https://www.sciencedirect.com/science/article/pii/S0047248417300507 |journal=Journal of Human Evolution |language=en |volume=105 |pages=24–40 |doi=10.1016/j.jhevol.2017.01.018 |pmid=28366198 |issn=0047-2484|url-access=subscription }}</ref> 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. <ref name="Russo and Kirk223" />  Comparing this to other animals, such as some primates, the foramen magnum are located more of a posterior position in the cranium.<ref name=":13" /> 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.<ref name=":13" /> 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]].

== Additional images ==
<gallery>
File:Medulla oblongata and foramen magnum animation small.gif|Skull seen from below. The hole through which the [[Medulla oblongata|medulla]] (shown in red) is passing is foramen magnum.
File:Occipital bone Opisthion01.png|Opisthion shown in red
File:Occipital bone Basion05.png|Occipital bone inner surface (basion shown in red)
</gallery>

== See also ==
* [[Posterior cranial fossa]]

== References ==
{{Gray's}}
{{Reflist}}

== External links ==
* {{Commons category-inline|Foramen magnum}}
* {{cite web|url=http://www.tk.de/rochelexikon/pics/s34257.000-1.html|title=Anatomy diagram: 34257.000-1|work= Roche Lexicon - illustrated navigator|publisher= Elsevier|archive-url=https://web.archive.org/web/20120722052131/http://www.tk.de/rochelexikon/pics/s34257.000-1.html|archive-date=2012-07-22}}
* [http://www.uni-mainz.de/FB/Medizin/Anatomie/makro1/201o004i.htm Diagram 1] {{Webarchive|url=https://web.archive.org/web/20160304201343/http://www.uni-mainz.de/FB/Medizin/Anatomie/makro1/201o004i.htm |date=2016-03-04 }}
* [http://www.uni-mainz.de/FB/Medizin/Anatomie/makro1/201o004u.htm Diagram 2] {{Webarchive|url=https://web.archive.org/web/20160304072848/http://www.uni-mainz.de/FB/Medizin/Anatomie/makro1/201o004u.htm |date=2016-03-04 }}
*{{YouTube|IClbdCNdjN8|3D animation showing position of basion}}
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{{Foramina of skull}}
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[[Category:Foramina of the skull]]