Bone of the neurocranium
The
occipital bone
(
) is a
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 overlies the
occipital lobes
of the
cerebrum
. At the
base of the skull
in the occipital bone, there is a large oval opening called the
foramen magnum
, which allows the passage of the
spinal cord
.
Like the other cranial bones, it is classed as a
flat bone
. Due to its many attachments and features, the occipital bone is described in terms of separate parts. From its front to the back is the
basilar part
, also called the basioccipital, at the sides of the foramen magnum are the
lateral parts
, also called the exoccipitals, and the back is named as the
squamous part
. The basilar part is a thick, somewhat quadrilateral piece in front of the foramen magnum and directed towards the
pharynx
. The squamous part is the curved, expanded plate behind the foramen magnum and is the largest part of the occipital bone.
Due to its embryonic derivation from paraxial mesoderm (as opposed to neural crest, from which many other craniofacial bones are derived), it has been posited that "the occipital bone as a whole could be considered as a giant vertebra enlarged to support the brain."
[1]
Structure
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The occipital bone, like the other seven
cranial bones
, has
outer and inner layers
(also called
plates
or
tables
) of
cortical bone tissue
between which is the
cancellous bone tissue
known in the cranial bones as
diploe
. The bone is especially thick at the ridges, protuberances,
condyles
, and anterior part of the
basilar part
; in the
inferior cerebellar fossae
it is thin, semitransparent, and without diploe.
Outer surface
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Near the middle of the outer surface of the squamous part of the occipital (the largest part) there is a prominence ? the
external occipital protuberance
. The highest point of this is called the
inion
.
From the inion, along the midline of the squamous part until the foramen magnum, runs a ridge ? the
external occipital crest
(also called the medial nuchal line) and this gives attachment to the
nuchal ligament
.
Running across the outside of the occipital bone are three curved lines and one line (the medial line) that runs down to the foramen magnum. These are known as the
nuchal lines
which give attachment to various ligaments and muscles. They are named as the
highest
,
superior
and
inferior
nuchal lines. The inferior nuchal line runs across the midpoint of the
median
nuchal line. The area above the highest nuchal line is termed the
occipital plane
and the area below this line is termed the
nuchal plane
.
Inner surface
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The inner surface of the occipital bone forms the base of the
posterior cranial fossa
. The
foramen magnum
is a large hole situated in the middle, with the
clivus
, a smooth part of the occipital bone travelling upwards in front of it. The median internal occipital crest travels behind it to the
internal occipital protuberance
, and serves as a point of attachment to the
falx cerebri
.
To the sides of the
foramen
sitting at the junction between the lateral and base of the occipital bone are the
hypoglossal canals
. Further out, at each junction between the occipital and
petrous portion of the temporal bone
lies a
jugular foramen
.
The inner surface of the occipital bone is marked by dividing lines as shallow ridges, that form four
fossae
or depressions. The lines are called the
cruciform
(cross-shaped)
eminence
.
At the midpoint where the lines intersect a raised part is formed called the
internal occipital protuberance
. From each side of this eminence runs a
groove
for the
transverse sinuses
.
There are two midline
skull landmarks
at the
foramen magnum
. The
basion
is the most anterior point of the opening and the
opisthion
is the point on the opposite posterior part. The basion lines up with the
dens
.
Foramen magnum
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The foramen magnum (
Latin
:
large hole
) is a large oval
foramen
longest front to back; it is wider behind than in front where it is encroached upon by the
occipital condyles
. The clivus, a smooth bony section, travels upwards on the front surface of the foramen, and the median internal occipital crest travels behind it.
Through the foramen passes the
medulla oblongata
and its membranes, the
accessory nerves
, the
vertebral arteries
, the
anterior
and
posterior spinal arteries
, and the
tectorial membrane
and
alar ligaments
.
Angles
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The
superior angle
of the occipital bone articulates with the occipital angles of the
parietal bones
and, in the fetal skull, corresponds in position with the
posterior fontanelle
.
The
lateral angles
are situated at the extremities of the
groove for the transverse sinuses
: each is received into the interval between the
mastoid angle of the parietal bone
, and the
mastoid portion of the temporal bone
.
The
inferior angle
is fused with the
body of the sphenoid bone
.
Borders
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The
superior borders
extend from the superior to the lateral angles: they are deeply serrated for articulation with the occipital borders of the parietals, and form by this union the
lambdoidal suture
.
The
inferior borders
extend from the lateral angles to the inferior angle; the upper half of each articulates with the mastoid portion of the corresponding
temporal
, the lower half with the
petrous
part of the same bone.
These two portions of the inferior border are separated from one another by the
jugular process
, the notch on the anterior surface of which forms the posterior part of the
jugular foramen
.
Sutures
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-
Lambdoid suture
-
Occipitomastoid suture
The
lambdoid suture
joins the occipital bone to the
parietal bones
.
The
occipitomastoid suture
joins the occipital bone and
mastoid portion
of the
temporal bone
.
The
sphenobasilar suture
joins the basilar part of the occipital bone and the back of the
sphenoid bone
body .
The
petrous-basilar suture
joins the side edge of the basilar part of the occipital bone to the
petrous-part
of the
temporal bone
.
Development
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The
occipital plane
[Fig. 3] of the squamous part of the occipital bone is developed in
membrane
, and may remain separate throughout life when it constitutes the
interparietal bone
; the rest of the bone is developed in cartilage.
The number of nuclei for the occipital plane is usually given as four, two appearing near the middle line about the second month, and two some little distance from the middle line about the third month of
fetal life
.
The
nuchal plane
of the squamous part is ossified from two centers, which appear about the seventh week of fetal life and soon unite to form a single piece.
Union of the upper and lower portions of the squamous part takes place in the third month of fetal life.
An occasional centre (
Kerckring
) appears in the posterior margin of the foramen magnum during the fifth month; this forms a separate ossicle (sometimes double) which unites with the rest of the squamous part before birth.
Each of the lateral parts begins to
ossify
from a single center during the eighth week of fetal life. The
basilar
portion is ossified from two centers, one in front of the other; these appear about the sixth week of fetal life and rapidly coalesce.
The occipital plane is said to be ossified from two centers and the basilar portion from one.
About the fourth year the squamous part and the two lateral parts unite, and by about the sixth year the bone consists of a single piece. Between the 18th and 25th years the occipital and
sphenoid bone
become united, forming a single bone.
Clinical significance
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Trauma to the occiput can cause a fracture of the base of the skull, called a
basilar skull fracture
. The
basion-dens line
as seen on a
radiograph
is the distance between the basion and the top of the dens, used in the diagnosis of dissociation injuries.
[4]
Genetic disorders
can cause a prominent occiput as found in
Edwards syndrome
, and
Beckwith?Wiedemann syndrome
.
The identification of the location of the
fetal
occiput is important in
delivery
.
Etymology
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Occipital stems from Latin
occiput
"back of the skull", from ob "against, behind" + caput "head". Distinguished from
sinciput
(anterior part of the skull).
[5]
Other animals
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In many animals these parts stay separate throughout life; for example, in the dog as four parts:
squamous part
(supraoccipital);
lateral parts
?left and right parts (exoccipital);
basilar part
(basioccipital).
The occipital bone is part of the
endocranium
, the most basal portion of the skull. In
Chondrichthyes
and
Agnatha
, the occipital does not form as a separate element, but remains part of the
chondrocranium
throughout life. In most
higher vertebrates
, the foramen magnum is surrounded by a ring of four bones.
The basioccipital lies in front of the opening, the two exoccipital condyles lie to either side, and the larger supraoccipital lies to the posterior, and forms at least part of the rear of the cranium. In many
bony fish
and
amphibians
, the supraoccipital is never ossified, and remains as
cartilage
throughout life. In primitive forms the basioccipital and exoccipitals somewhat resemble the centrum and neural arches of a
vertebra
, and form in a similar manner in the embryo. Together, these latter bones usually form a single concave circular
condyle
for the articulation of the first
vertebra
.
[6]
In
mammals
, however, the condyle has divided in two, a pattern otherwise seen only in a few amphibians.
Most mammals also have a single fused occipital bone, formed from the four separate elements around the foramen magnum, along with the paired
postparietal
bones that form the rear of the
cranial roof
in other vertebrates.
[6]
Additional images
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See also
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References
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Books
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This article incorporates text in the
public domain
from
page 129
of the 20th edition of
Gray's Anatomy
(1918)
- Susan Standring; Neil R. Borley; et al., eds. (2008).
Gray's anatomy: the anatomical basis of clinical practice
(40th ed.). London: Churchill Livingstone.
ISBN
978-0-8089-2371-8
.
Citations
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External links
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