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Occipital Bone and Foramen Magnum

Surgical Correlation

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The occipital bone surrounds the oval-shaped fora-men magnum, which is wider posteriorly than anteriorly. The narrower anterior part sits above the odontoid process and it encroached on from laterally by the occipital condyles. The wider posterior part transmits the medulla. The occipital bone is divided into a squamosal part located above and behind the foramen magnum; a basal (clival) part situated in front of the foramen magnum; and paired condylar parts located lateral to the foramen magnum. The squamous part is internally concave. Its upper margin articulates with the parietal bone at the lambdoid suture, and its lower margin articulates with the mastoid portion of the temporal bone at the occipitomastoid suture. The convex external surface of the squamosal part has several prominences. The largest prominence, the external occipital protuberance (inion), is situated at the central part of the external surface. The superior nuchal line radiates laterally from the protuberance. A vertical ridge, the external occipital crest, descends from the external occipital protuberance to the midpoint of the posterior margin of the foramen magnum. The inferior nuchal lines run laterally on both sides from the midpoint of the crest. The internal surface of the squamous part is concave and has a prominence, the internal occipital protuberance, near its center. The internal surface is divided into four unequal fossae by the sulcus of the superior sagittal sinus, the internal occipital crest, and the sulci for the transverse sinuses. The internal occipital crest bifurcates above the foramen magnum to form a V-shaped ridge between the limbs of which is the vermian fossa. The basilar part of the occipital bone, which is also referred to as the clivus, is a thick quadrangular plate of bone that extends forward and upward to join the sphenoid bone just below the dorsum sellae. The superior surface of the clivus slopes upward from the foramen magnum and is concave from side to side. The clivus is separated on each side from the petrous part of the temporal bone by the petroclival fissure that ends posteriorly at the jugular foramen. The occipitomastoid suture extends posterolateral from the jugular foramen. On the inferior surface of the basilar part, a small elevation, the pharyngeal tubercle, gives attachment to the fibrous raphe of the pharynx. The condylar parts of the occipital bone, on which the occipital condyles are located, are situated lateral to the foramen magnum on the external surface. The alar tubercle, which gives attachment to the alar ligament, is situated on the medial side of each condyle. The hypoglossal canal is situated above the condyle. The condylar fossa, which may be converted into a foramen for the passage of an emissary vein, is located behind the condyle. The jugular process of the occipital bone extends laterally from the posterior half of the condyle and articulates with the jugular surface of the temporal bone. The sulcus of the sigmoid sinus crosses the superior surface of the jugular process. The jugular foramen is bordered posteriorly by the jugular process of the occipital bone and anteriorly by the jugular fossa of the petrous temporal bone. The jugular tubercle lies on the internal surface above the hypoglossal canal. A, Inferior view. B, Posteroinferior view. C, Anterior-inferior view. D, Superior view. E, Posterosuperior view. F, Oblique posterosuperior view. (Images courtesy of AL Rhoton, Jr.)

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