A, Posterior superior view of the jugular foramen. The sigmoid sulcus makes a sharp turn just before emptying into the sigmoid portion of the jugular foramen. The inferior petrosal sinus extends along the petroclival fissure and enters the petrosal part of the foramen. The nerves enter the intrajugular part of the foramen located between the sigmoid and petrosal parts. The outlined area shows the approximate site from which B to F were taken. B, The sigmoid sinus descends in the sigmoid sulcus and makes a sharp anterior turn to enter the jugular foramen. The jugular bulb extends upward under the petrous temporal bone toward the internal acoustic meatus. The endolymphatic sac is located above the lower portion of the sigmoid sinus on the back of the temporal bone and opens above through the vestibular aqueduct into the vestibule. The glossopharyngeal, vagus and accessory nerves penetrate the dura on the medial side of the intrajugular process. C, The dura covering the jugular foramen and the jugular bulb have been removed. The nerves penetrate the dura on the medial side of the intrajugular process of the temporal bone. The intrajugular ridge extends forward along the medial side of the jugular bulb. D, Enlarged view. The glossopharyngeal nerve passes forward along the medial side of the intrajugular ridge, but the vagus and accessory nerves, although entering the dura on the medial side of the intrajugular process, almost immediately turn downward and do not course along the medial edge of the intrajugular ridge in the medial wall of the jugular bulb, as does the glossopharyngeal nerve. The auricular branch of the vagus nerve (Arnold’s Nerve) arises from the vagus nerve, passes along the groove in the anterior wall of the jugular fossa, and penetrates the mastoid canaliculus in the lateral wall of the fossa. E, The nerves entering the jugular foramen have been displaced downward. The intrajugular process of the temporal bone projects backward to join the intrajugular process of the occipital bone, thus forming an osseous bridge that divides the foramen into two parts. The vagus and accessory nerves pass lateral to the osseous bridge and the inferior petrosal sinus descends below the bridge to open into the internal jugular vein. F, The hypoglossal nerve has been exposed on the lateral side of the occipital condyle. It exits the hypoglossal canal and joins the glossopharyngeal, vagus, and accessory nerves below the jugular foramen in the interval between the internal carotid artery and internal jugular vein. (Images courtesy of AL Rhoton, Jr.)