Anatomy of the Posterior Skull Base

The clivus is located in the middle area of the posterior cranial fossa. Its upper portion develops from the sphenoid bone, while its lower portion is the basilar part of the occipital bone. Normally, the spheno-occipital synchondrosis closes between 13.5 and 18.5 years starting at the intracranial end and progressing to the outer skull base area. The total length of the clivus in our specimens was measured to be 45.9 range (37-52 mm). Holsten and Herrmann (1970) estimated the normal length of the basilar part of the clivus to be 32 mm, a value which compares well with our measurements of this structure during postnatal development. A shortened pars basilaris of the occipital bone has also been described and called occipital hyppplasia: this structure was approx. 24 mm in length 43• The pars lateralis of the occipital bone in childhood is bordered anteriorly by the anterior intraoccipital synchondrosis and posteriorly by the posterior intraoccipital synchondrosis. The posterior synchondrosis closes earlier than the anterior one, which is situated in the area of the anterior border of the hypoglossal canal. Behind the pars lateralis of the occipital bone, the squama occipitalis forms the posterior border of the foramen magnum and a large part of the floor of the posterior cranial fossa. Anteriorly and laterally, the petrous part of the temporal bone is connected to the squama of the occipital bone. Between the basilar part of the occipital bone and the petrous bone is situated the synchondrosis petro-occipitalis. In the synchondrosis small , ossicles are sometimes developed~ In this area, cartilage is also found in the skulls of children and young adults; in specimens from older individuals we noted first sutures, than synostoses between the two bone parts. This area also includes the inferior petrosal sinus, which also extends to the jugular foramen area. In anatomical terms, this is called the sulcus sinus petrosi inferioris, of the occipital bone; there is also a sulcus sinus petrosi inferioris of the temporal bone. Occasionally the petrous temporal bones in our specimens had such a sulcus only in the upper area (sometimes it was absent) and the whole sulcus was grooved in the basilar part of the occipital bone. In the lower part, the suture between the occipital and petrous bones is situated in most cases near the petrous bone. The posterior part of the posterior skull base is usually thickened by the internal occipital crest, and above this crest by the internal occipital protuberance. Near the foramen magnum a flattened or hollowed ara (Vermian impression or triangle) can be seen in most cases. Laterally are the cerebellar fossae, on which lie the lower surfaces of the cerebellar hemispheres, separated from the bone by dura and subarachnoid space. Other important landmarks are the grooves for the lateral and sigmoid sinuses. In approx. 3% of our specimens, the occipital sinus (normally embedded in the falx cerebelli) had two limbs arranged in a V-shaped pattern around the foramen magnum and running toward the jugular foramen. This arrangement was found more often in children than in adults 15 • These sinuses were described as oblique occipital sinuses. The thickest and thinnest zones of the posterior part of the posterior cranial base, its sutures in adults, the mastoid emissary vein, and some