Opening in the occipital bone of the skull
The
foramen magnum
(
Latin
:
great hole
) is a large, oval-shaped opening in the
occipital bone
of the
skull
. 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.
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 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
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]
Structure
[
edit
]
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]
Variation
[
edit
]
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
).
Heart-shaped foramen magnum
[
edit
]
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
.
[7]
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.
[7]
The Hadar shows traits and characteristics of having a similar cardioid shape.
[7]
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.
[7]
With the enlargement and broadening of the anterior foramen magnum, allow for more venous drainage from the occipital and marginal sinuses.
[7]
Function
[
edit
]
The foramen magnum transmits a number of important structures between the neck and the neurocranium.
Structures passing through anterior compartment (osseoligamentous compartment) include:
Structures passing through posterior compartment (neurovascular compartment) include:
Clinical significance
[
edit
]
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]
Other animals
[
edit
]
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]
Evolution of foramen magnum in fossils
[
edit
]
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
.
Additional images
[
edit
]
See also
[
edit
]
References
[
edit
]
This article incorporates text in the
public domain
from
page 129
of the 20th edition of
Gray's Anatomy
(1918)
- ^
Russo, Gabrielle A.; Kirk, Christopher E. (November 2013). "Foramen magnum position in bipedal mammals".
Journal of Human Evolution
.
65
(5): 656?670.
CiteSeerX
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.
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f
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Sanchez, Pedro; Graham, John M. (2017).
"31 - Congenital Anomalies of the Skull"
.
Swaiman's Pediatric Neurology
(6th ed.).
Elsevier
. pp. 233?241.
doi
:
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.
ISBN
978-0-323-37101-8
.
- ^
Dutta, Asim Kumar (2013).
Essentials of Human Anatomy Head & Neck
. kolkata: Current books international. pp. 56?57.
ISBN
978-81-86793-79-4
.
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"Another look at the foramen magnum in bipedal mammals"
.
Journal of Human Evolution
.
105
: 24?40.
doi
:
10.1016/j.jhevol.2017.01.018
.
ISSN
0047-2484
.
PMID
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.
- ^
Sanchez, Pedro; Graham, John M. (2017).
"31 - Congenital Anomalies of the Skull"
.
Swaiman's Pediatric Neurology
(6th ed.).
Elsevier
. pp. 233?241.
doi
:
10.1016/B978-0-323-37101-8.00031-X
.
ISBN
978-0-323-37101-8
.
- ^
Zdilla, Matthew J; Russell, Michelle L; Bliss, Kaitlyn N; Mangus, Kelsey R; Koons, Aaron W (2017).
"The size and shape of the foramen magnum in man"
.
Journal of Craniovertebral Junction & Spine
.
8
(3): 205?221.
doi
:
10.4103/jcvjs.JCVJS_62_17
.
ISSN
0974-8237
.
PMC
5634107
.
PMID
29021672
.
- ^
a
b
c
d
e
Tobias, Phillip V.; Symons, Joel A. (1992).
"Functional, Morphogenetic and Phylogenetic Significance of Conjunction between Cardioid Foramen Magnum and Enlarged Occipital and Marginal Venous Sinuses"
.
Archaeology in Oceania
.
27
(3): 120?127.
doi
:
10.1002/j.1834-4453.1992.tb00295.x
.
ISSN
0728-4896
.
JSTOR
40386950
.
- ^
a
b
Russo, Gabrielle A.; Kirk, E. Christopher (2013-11-01).
"Foramen magnum position in bipedal mammals"
.
Journal of Human Evolution
.
65
(5): 656?670.
doi
:
10.1016/j.jhevol.2013.07.007
.
ISSN
0047-2484
.
PMID
24055116
.
- ^
a
b
Russo, Gabrielle A.; Kirk, E. Christopher (2013-11-01).
"Foramen magnum position in bipedal mammals"
.
Journal of Human Evolution
.
65
(5): 656?670.
doi
:
10.1016/j.jhevol.2013.07.007
.
ISSN
0047-2484
.
PMID
24055116
.
- ^
a
b
c
Russo, Gabrielle A.; Kirk, E. Christopher (2017-04-01).
"Another look at the foramen magnum in bipedal mammals"
.
Journal of Human Evolution
.
105
: 24?40.
doi
:
10.1016/j.jhevol.2017.01.018
.
ISSN
0047-2484
.
PMID
28366198
.
External links
[
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]