Occipital Bone: Structure, Function & Diseases

The occipital bone (Os occipitale) is a part of the brain skull. The bone consists of three parts and not only contains various openings, but also serves as an attachment site for tissues. The occipital bone can fracture in basilar skull fracture, and trisomy 18 often results in a large occipital bone.

What is the occipital bone?

The bones of the skull capsule form a rounded vault that contains the brain. They provide support for the soft tissue of the complex organ and shield it from direct contact with the environment. The occipital bone is one of the bones that belong to the cerebral skull (neurocranium). In total, the cerebral skull counts seven different bones, and the entire skull – including the facial skull – includes 22. The occipital bone is located at the back of the head, where it is found between the sphenoid bone (Os sphenoidale), temporal bone (Os temporale) and parietal bone (Os parietale). In anatomy, the occipital bone is also known by the technical term “Os occipitale”. Like all bones, the flat skull bone consists of a framework of tissue that only hardens completely in the course of physical development.

Anatomy and structure

The occipital bone is composed of three parts that are normally fused together: the pars squamosa, the pars lateralis, and the pars basilaris. The pars squamosa lies below (dorsal) the foramen magnum. The foramen magnum is a large opening in the cranium through which the medulla oblongata exits the posterior fossa and passes into the spinal cord. The pars squamosa has a bowl-shaped form and develops from two subunits. The occipital plate develops from four centers, from where the bone tissue grows together. In contrast, the nuchal plate of the pars squamosa develops from two nuclei from the seventh week of development. The pars lateralis forms the lateral portions of the occiput and develops from one nucleus about one week later. On each side, the pars lateralis has a condyle occipitalis, which is a component of the atlantooccipital joint (articulatio atlantooccipitalis). The pars basilaris forms the part of the occipital bone that rostrally closes the skull toward the middle of the head. It has approximately the shape of a quadrilateral and also arises from a center during physical development.

Function and tasks

As part of the cerebral skull, the occipital bone has the function of supporting and shielding the brain. It also contains or provides support for numerous structures. Together with the temporal bone, the occipital bone forms the posterior fossa. It contains the cerebellum, the midbrain, the bridge and the medulla oblongata. The latter protrudes through the foramen magnum, which is located at the base of the occipital bone. The pars squamosa has bony prominences and depressions. One such depression is the sulcus sinus transversi, in which the sinus transversus runs. The transverse sinus is a blood conductor that drains venous blood from the skull. Another depression in the pars squamosa of the occipital bone is the sulcus sinus sigmoidei. It contains the sigmoid sinus, another venous blood conductor. The two sulci are located on the inner side of the pars squamosa. There, the protuberantia occipitalis interna forms a small protrusion where there is an attachment of the cerebral crescent (falx cerebri). The skin separates the two cerebral hemispheres. On the outside of the pars squamosa, the protuberantia occipitalis externa also provides an attachment point for the hood or trapezius muscle (Musculus trapezius). At the pars lateralis of the occipital bone, the skull is connected to the atlas via the atlantooccipital joint. The atlas represents the uppermost cervical vertebra (C1) and thus forms the beginning of the spinal column. On the inner side of the pars lateralis lies the tuberculum jugulare, which is a bony prominence covering the hypoglossal canal. In some cases, the tuberculum jugulare also provides a depression for cranial nerves IX-XI. With the help of its processus jugularis, the pars lateralis also functions as the attachment point of a neck muscle, the musculus rectus capitis lateralis. Furthermore, the occiput forms an internal protrusion at the pars lateralis known as the tuberculum pharyngis. This is where the rectus capitis anterior muscle, the pharyngeal suture (raphe pharyngis) and the longus capitis muscle attach. The clivus of the pars lateralis forms the boundary between the posterior cranial fossa and the media cranial fossa.

Diseases

Injuries to the head can result in a basal skull fracture, which often involves the occipital bone as well. Medical science distinguishes between a frontobasal fracture involving the nose and a laterobasal fracture in which the temporal bone also breaks. Possible symptoms include monocular/glandular hematoma, leakage of cerebrospinal fluid and blood, and impaired consciousness. When cranial nerves or parts of the brain are damaged, additional neurological symptoms may occur, such as those indicating cranial nerve failure. However, some signs of illness may also resemble the clinical picture of a stroke. In some cases, the skull base fracture causes bleeding around the eye. Affected persons may feel pulsation in the eye or the eyeball may protrude forward due to the swelling. Medical professionals then refer to this as exophthalmos or protrusio bulbi. In connection with trisomy 18, the occipital bone of affected individuals is often conspicuously developed. The genetic disorder is also known as Edwards syndrome and can manifest itself in very different ways. Typical are above all malformations and short stature. In most cases (around 90%), trisomy 18 leads to death before birth and the mortality of children born with Edwards syndrome is also very high. Treatment usually focuses on the symptoms, since medicine cannot treat the cause of the genetic disease. In some cases, supportive measures are necessary, such as artificial feeding.