The foramen magnum () is a large, oval-shaped opening in the

occipital bone The occipital bone () is a neurocranium, cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lob ...

of the

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

. It is one of the several oval or circular openings (foramina) in the

base of the skull The base of skull, also known as the cranial base or the cranial floor, is the most inferior area of the skull. It is composed of the endocranium and the lower parts of the calvaria. Structure Structures found at the base of the skull are for ...

. The

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue that extends from the medulla oblongata in the lower brainstem to the lumbar region of the vertebral column (backbone) of vertebrate animals. The center of the spinal c ...

, an extension of the

medulla oblongata The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involun ...

, passes through the foramen magnum as it exits the

cranial cavity The cranial cavity, also known as intracranial space, is the space within the skull that accommodates the brain. The skull is also known as the cranium. The cranial cavity is formed by eight cranial bones known as the neurocranium that in human ...

. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the vertebral arteries, the

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

and posterior spinal arteries, the tectorial membranes and alar ligaments. It also transmits the

accessory nerve The accessory nerve, also known as the eleventh cranial nerve, cranial nerve XI, or simply CN XI, is a cranial nerve that supplies the sternocleidomastoid and trapezius muscles. It is classified as the eleventh of twelve pairs of cranial nerv ...

into the skull. The foramen magnum is a very important feature in

bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...

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 The Hominini (hominins) form a taxonomic tribe of the subfamily Homininae (hominines). They comprise two extant genera: ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos), and in standard usage exclude the genus ''Gorilla'' (gorillas), ...

, including modern humans, ''Australopithecus africanus'', and ''Paranthropus boisei''. This common feature of bipedal

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.

Structure

The foramen magnum is a large, oval-shaped opening (

foramen In anatomy and osteology, a foramen (; : foramina, or foramens ; ) is an opening or enclosed gap within the dense connective tissue (bones and deep fasciae) of extant and extinct amniote animals, typically to allow passage of nerves, artery, ...

) in the occipital bone of the

. It is present in

humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

, and in many other

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...

. Anteriorly, it is bounded by the basiocciput. Posteriorly, it is bounded by the supraocciput. Laterally, it is bounded by the occipital condyles. 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.

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. In some rodents and mammals, the foramen magnum can be found anteriorly in the cranium. The foramen magnum varies in size between individuals. Earlier

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

of the occipital bone leads to a smaller foramen. 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. In humans, the foramen magnum is farther underneath the head than in the other

great apes The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...

. 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 Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) Limb (anatomy), limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from ...

) rather than four (

quadrupedalism Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...

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. 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. The Hadar shows traits and characteristics of having a similar cardioid shape. 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. With the enlargement and broadening of the anterior foramen magnum, allow for more venous drainage from the occipital and marginal sinuses.

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. * the upper band of

cruciate ligament Cruciate ligaments (also cruciform ligaments) are pairs of ligaments arranged like a letter X. They occur in several joints of the body, such as the knee joint, wrist joint and the atlanto-axial joint. In a fashion similar to the cords in ...

of the

atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...

( C1 vertebra). * the membrana tectoria. Structures passing through posterior compartment (neurovascular compartment) include: * the lower end of

, surrounded by

meninges In anatomy, the meninges (; meninx ; ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in the subarachnoid spac ...

. * the fourth part of the

vertebral artery The vertebral arteries are major artery, arteries of the neck. Typically, the vertebral arteries originate from the subclavian arteries. Each vessel courses superiorly along each side of the neck, merging within the skull to form the single, m ...

, surrounded by sympathetic plexus of nerves. *

s. * anterior and posterior spinal arteries. * the tonsil of the

cerebellum The cerebellum (: cerebella or cerebellums; Latin for 'little brain') is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as it or eve ...

(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. A small foramen magnum can cause neurological problems, and the reduced circulation of cerebrospinal fluid can cause

hydrocephalus Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up within the brain, which can cause pressure to increase in the skull. Symptoms may vary according to age. Headaches and double vision are common. Elderly adults with n ...

. This may be treated with suboccipital craniectomy.

Other animals

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

Evolution of foramen magnum in fossils

Although not fully proven, there are many studies that show the possibility 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. 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. Comparing this to other animals, such as some primates, the foramen magnum are located more of a posterior position in the cranium. 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. 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

Additional images

File:Medulla oblongata and foramen magnum animation small.gif, Skull seen from below. The hole through which the 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)

References

External links

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Diagram 1

* {{Authority control Foramina of the skull