The occipital bone (/ˌɒkˈsɪpɪtəl/) 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 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.
Human skull (Occipital bone is at bottom right).
Position of occipital bone (shown in green)
|Articulations||the two parietals, the two temporals, the sphenoid, and the atlas|
|Anatomical terms of bone|
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 diploë. 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 diploë.
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 medial 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.
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.
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.
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.
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.
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.
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 .
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.
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.
In many animals these parts stay separate throughout life; for example, in the dog as four parts: squamous part (supraoccipita); 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.
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.
The atlanto-occipital joint (articulation between the atlas and the occipital bone) consists of a pair of condyloid joints. The atlanto-occipital joint is a synovial joint.Basilar part of occipital bone
The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline.
In the young skull this area is rough and uneven, and is joined to the body of the sphenoid by a plate of cartilage.
By the twenty-fifth year this cartilaginous plate is ossified, and the occipital and sphenoid form a continuous bone.Condyloid fossa
Behind either condyle of the lateral parts of occipital bone is a depression, the condyloid fossa (or condylar fossa), which receives the posterior margin of the superior facet of the atlas when the head is bent backward; the floor of this fossa is sometimes perforated by the condyloid canal, through which an emissary vein passes from the transverse sinus.Cruciform eminence
The cruciform eminence (or cruciate eminence), divides the deeply concave internal surface of the occipital bone into four fossae:
The upper two fossae are called the cerebral fossae, are triangular and lodge the occipital lobes of the cerebrum.
The lower two are called the cerebellar fossae, are quadrilateral and accommodate the hemispheres of the cerebellum.The upper fossae are separated from the lower fossae by a groove for the transverse sinuses. At the point of intersection between all four fossae is the internal occipital protuberance.External occipital protuberance
Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of which is referred to as the inion. The inion is the most prominent projection of the protuberance which is located at the posterioinferior (lower rear) part of the human skull. The nuchal ligament and trapezius muscle attach to it.
The inion (ἰνίον, iníon, Greek for the occipital bone) is used as a land→÷mark in the 10-20 system in electroencephalography (EEG) recording. Extending laterally from it on either side is the superior nuchal line, and above it is the faintly marked highest nuchal line.
A study of 16th-century Anatolian remains showed that the external occipital protuberance statistically tends to be less pronounced in female remains.Groove for transverse sinus
Along the internal surface of the occipital bone, running laterally between the superior and inferior fossae of the cruciform eminence is the groove for transverse sinus. The transverse sinuses travel along this groove.Hypoglossal canal
The hypoglossal canal is a foramen in the occipital bone of the skull. It is hidden medially and superiorly to each occipital condyle. The hypoglossal nerve traverses the canal.Internal occipital crest
In the occipital bone, the lower division of the cruciate eminence is prominent, and is named the internal occipital crest; it bifurcates near the foramen magnum and gives attachment to the falx cerebelli; in the attached margin of this falx is the occipital sinus, which is sometimes duplicated.
In the upper part of the internal occipital crest, a small depression is sometimes distinguishable; it is termed the vermian fossa since it is occupied by part of the cerebellar vermis of the cerebellum.Intrajugular process of occipital bone
Intrajugular process of occipital bone (Processus intrajugularis ossis occipitalis) is a small, pointed process extending from the middle of the jugular notch of occipital bone, that subdivides the jugular notch of the occipital bone into a lateral and a medial part.Jugular process
In the lateral part of the occipital bone, extending lateralward from the posterior half of the condyle is a quadrilateral or triangular plate of bone, the jugular process, excavated in front by the jugular notch, which, in the articulated skull, forms the posterior part of the jugular foramen.
It serves as the insertion of the rectus capitis lateralis.Jugular tubercle
The upper surface of the lateral parts of occipital bone presents an oval eminence, the jugular tubercle, which overlies the hypoglossal canal and is sometimes crossed by an oblique groove for the glossopharyngeal, vagus, and accessory nerves.Lambdoid suture
The lambdoid suture (or lambdoidal suture) is a dense, fibrous connective tissue joint on the posterior aspect of the skull that connects the parietal bones with the occipital bone. It is continuous with the occipitomastoid suture.
Its name comes from its uppercase lambda-like shape.Lateral parts of occipital bone
The lateral parts of the occipital bone (also called the exoccipitals) are situated at the sides of the foramen magnum; on their under surfaces are the condyles for articulation with the superior facets of the atlas.Pharyngeal tubercle
The pharyngeal tubercle is a part of the occipital bone of the head and neck. It is located on the lower surface of the basilar part of occipital bone, about 1 cm. anterior to the foramen magnum. The pharyngeal tubercle gives attachment to the fibrous raphe of the pharynx, also known as the pharyngeal raphe.
This is the point of attachment for the superior pharyngeal constrictor muscle.Platybasia
Platybasia is a spinal disease of a malformed relationship between the occipital bone and cervical spine.
It may be caused by Paget's disease. Platybasia is also a feature of Gorlin-Goltz syndrome, commonly known as basal cell nevus syndrome.Rectus capitis lateralis muscle
The Rectus capitis lateralis, a short, flat muscle, arises from the upper surface of the transverse process of the atlas, and is inserted into the under surface of the jugular process of the occipital bone.Squamous part of occipital bone
The squamous part of occipital bone, is situated above and behind the foramen magnum, and is curved from above downward and from side to side.Suboccipital muscles
The suboccipital muscles are a group of muscles defined by their location to the occiput. Suboccipital muscles are located below the occipital bone. These are four paired muscles on the underside of the occipital bone; the two straight muscles (rectus) and the two oblique muscles (obliquus).
The muscles are named
Rectus capitis posterior major goes from the spinous process of the axis (C2) to the occipital bone.
Rectus capitis posterior minor goes from the middle of the posterior arch of the atlas to the occipit.
Obliquus capitis superior goes from the transverse process of the atlas to the occiput.
Obliquus capitis inferior goes from the spine of the axis vertebra to the transverse process of the atlas.They are innervated by the suboccipital nerve.Trapezius
The trapezius (or trapezoid) is a large paired surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It moves the scapula and supports the arm.
The trapezius has three functional parts: an upper (descending) part which supports the weight of the arm; a middle region (transverse), which retracts the scapula; and a lower (ascending) part which medially rotates and depresses the scapula. The trapezius is used to nod your head. It is also used to hold up your head.
General anatomy: systems and organs, regional anatomy, planes and lines, superficial axial anatomy, superficial anatomy of limbs