Internal Aspects of Base of Skull: Orifices


The arcuate eminence is produced by the superior semicircular canal of the internal ear. Anterolateral to this eminence is a thin plate of bone, the tegmen tympani, forming the roof of the tympanic cavity and mastoid antrum and extending forward and medially to cover the bony part of the auditory (pharyngotympanic) tube.


The posterior cranial fossa is the largest and deepest of the cranial fossae and lodges the cerebellum, pons, and medulla oblongata. It is bounded anteriorly by the dorsum sellae, the back of the body of the sphenoid bone, and the basilar part of the occipital bones; posteriorly by the squama of the occipital bone below the sulci for the transverse sinuses and the internal occipital protuberance; and laterally by the petrous and mastoid parts of the temporal bones, the mastoid angles of the parietal bones, and the lateral parts of the occipital bone.


The posterior fossa is pierced by a number of foramina and is grooved by various dural venous sinuses. A large median opening in the floor of the fossa, the foramen magnum, penetrates the occipital bone. The medulla oblongata and spinal cord and their surrounding meninges become directly continuous immediately below the foramen. The petrous part of the temporal bone and the occipital bone are separated by the petro-occipital fissure and the sulcus for the inferior petrosal sinus; the fissure ends behind, in the jugular foramen. The inferior petrosal and sigmoid sinuses pass through the anterior and posterior parts of this foramen, respectively, while the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves occupy an intermediate position as they leave the skull.


Two canals are associated with the occipital condyles: the hypoglossal canal, for the twelfth cranial nerve, and the condylar canal.


Above the jugular foramen, the internal acoustic meatus tunnels into the petrous part of the temporal bone. It is about 1 cm long and is separated laterally from the internal ear by a thin bony plate pierced by many apertures for fascicles of the facial (VII) and vestibulocochlear (VIII) nerves. Behind the orifice of this meatus is the slitlike opening of the vestibular aqueduct, which lodges the blind end of the endolymphatic duct.


The internal opening of the inconstant mastoid foramen is close to the sulcus for the sigmoid sinus, which winds downward from the transverse sinus to the jugular foramen, where it ends in the superior bulb of the internal jugular vein. The internal occipital protuberance is related to the confluence of the superior sagittal, straight, occipital, and transverse sinuses. The margins of the sulci for the transverse sinuses give attachment to the tentorium cerebelli.


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Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Internal Aspects of Base of Skull: Orifices

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