The occipital bone makes up the posterior portion of the cranium and the skull base and contains three parts: the squamous part, basilar part, and lateral parts. The occipital bone articulates with the parietal (lambdoid suture), temporal (occipitomastoid suture), and sphenoid bones. Externally, the most prominent part of the squamous portion of the occipital bone is the external occipital protuberance, specifically the inion, to which the nuchal ligament and trapezius muscles attach. The planum occipitale is the smooth portion of the bone superiorly.
Inferior to the planum occipitale are a series of nuchal lines, the superior and inferior nuchal lines oriented transversely. The superior nuchal lines join medially to the external occipital protuberance. The median nuchal line extends from the external occipital protuberance to the foramen magnum. The interior surface of the squamous part contains the internal occipital protuberance, occupied by the torcular Herophili, which is the junction of the sagittal sulcus, grooves of the transverse sinuses, and the occipital sulcus. The vermian fossa lies in the posterior portion of the foramen magnum.
The basilar part of the occipital bone extends upward from the foramen magnum forming the clivus, which articulates with the dorsum sellae of the sphenoid bone. The exterior surface of the basilar part contains the pharyngeal tubercle.
The lateral parts of the occipital bone make up the sides of the foramen magnum. On their undersurface lie the occipital condyles. Behind the occipital condyle is the condyloid fossa and condyloid canal, which transmits an emissary vein. The hypoglossal canal is a tunnel within the condyle which transmits the hypoglossal nerve (XII) and the meningeal branch of the ascending pharyngeal artery.
The hypoglossal canal is an important landmark for far lateral approaches to the ventral brainstem. On the external surface, extending laterally from the condyle, is the jugular process with the jugular notch anterior to it. The jugular notch makes the posterior part of the jugular foramen. The upper surface of the lateral part forms the jugular tubercle which overlies the hypoglossal canal. The largest foramen in the occipital bone, the foramen magnum, transmits the medulla, the spinal accessory nerve (XI), vertebral arteries, anterior spinal arteries, posterior spinal arteries, and alar ligaments.