The Occipital Bone
The occipital bone is an unpaired bone which covers the back of the head (occiput). It makes up a large portion of the basilar part of the neurocranium and houses the cerebellum. It is the only cranial bone to articulate with the cervical spine.
The bone is convex externally and concave internally. It is divided into four parts: the basilar part, two condylar parts and the squamous part. All four are arranged around a large opening, the foramen magnum. Both the brainstem (medulla oblongata) and spinal branch of accessory nerve as well as important vessels such as the anterior and posterior spinal arteries, vertebral artery and spinal vein pass through it. The basilar part sits anterior to the foramen magnum and adjacent to the petrous part of the temporal bone. Anteriorly it fuses with the sphenoid bone to form the clivus during adolescence (tribasilar bone). Both the superior pharyngeal constrictor muscle and the pharyngeal raphe insert onto the pharyngeal tubercle found on the inferior surface of the basilar part.
The condylar parts are located lateral to the foramen magnum. They comprise two kidney-shaped prominences (occipital condyles) that articulate with the first cervical vertebra (atlanto-occipital joint). Posterior to the them are the condylar canals where the condylar emissary veins pass through and connect the external vertebral venous plexuses with the sigmoid sinuses. The hypoglossal nerve exits through the hypoglossal canal which pierces through the condylar part of the occipital.
The squamous part is the largest of all four. A palpable prominence known as the external occipital protuberance lies on the midline of the external surface which serves as an attachment for the trapezius muscle. Furthermore the external surface features three curved lines referred to as nuchal lines:
- The highest nuchal line extends laterally from the external occipital protuberance and is the site of origin of the epicranius muscle and epicranial aponeurosis.
- The superior nuchal line runs slightly inferior. It provides origin to the trapezius, sternocleidomastoid and splenius capitis muscles.
- The inferior nuchal line runs further inferiorly. The semispinal capitis muscle inserts above it.
The squamous part of the occipital bone is marked by grooves on its internal surface due to dural venous cranial sinuses: the superior sagittal sinus, the transverse sinuses and the sigmoid sinus. Superior to the groove for transverse sinus is a depression that accommodates the occipital brain lobes (cerebral fossa) and the corresponding inferior depression houses the cerebellum (cerebellar fossa).
The occipital bone is bordered superiorly and laterally by the lambdoid suture which separates it from the parietal bones. It articulates with the mastoid process through the occipitomastoid suture. The petro-occipital suture joints the petrous part of the temporal bone with the occipital bone. The spheno-occipital suture between the sphenoid and occipital bones disappears as they fuse during adolescence.
The basilar and condylar parts develop through endochondral ossification. Most of the squamous part undergoes endochondral ossification as well except its superior part which develops through the intramembranous pathway. In newborns all four parts are separated by connective tissue. By the age of four the squamous and condylar parts will usually have ossified together whereas the basilar portion joins them at age six.
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