Also passing through the jugular foramen are the glossopharyngeal nerve [IX], the vagus nerve [X], and the accessory nerve [XI].
Medial to the jugular foramen is a large rounded mound of the occipital bone (the jugular tubercle). Just inferior to this, and superior to the foramen magnum, is the hypoglossal canal, through which the hypoglossal nerve [XII] leaves the posterior cranial fossa, and a meningeal branch of the ascending pharyngeal artery enters the posterior cranial fossa.
Just posterolateral to the hypoglossal canal is the small condylar canal that, when present, transmits an emissary vein.
Squamous part of the occipital bone
The squamous part of the occipital bone has several prominent features (Fig. 8.27):
Running upward in the midline from the foramen magnum is the internal occipital crest.
On either side of the internal occipital crest, the floor of the posterior cranial fossa is concave to accommodate the cerebellar hemispheres.
The internal occipital crest ends superiorly in a bony prominence (the internal occipital protuberance).
Extending laterally from the internal occipital protuberance are grooves produced by the transverse sinuses, which continue laterally, eventually joining a groove for each sigmoid sinus—each of these grooves then turns inferiorly toward the jugular foramina.
The transverse and sigmoid sinuses are intradural venous sinuses.
Foramina and fissures through which major structures enter and leave the cranial cavity
Foramina and fissures through which major structures pass between the cranial cavity and other regions are summarized in Fig. 8.28.
The brain, as well as the spinal cord, is surrounded by three layers of membranes (the meninges, Fig. 8.31A)—a tough, outer layer (the dura mater), a delicate, middle layer (the arachnoid mater), and an inner layer firmly attached to the surface of the brain (the pia mater).
The cranial meninges are continuous with, and similar to, the spinal meninges through the foramen magnum, with one important distinction—the cranial dura mater consists of two layers, and only one of these is continuous through the foramen magnum (Fig. 8.31B).
The cranial dura mater is a thick, tough, outer covering of the brain. It consists of an outer periosteal layer and an inner meningeal layer (Fig. 8.31A):
The outer periosteal layer is firmly attached to the skull, is the periosteum of the cranial cavity, contains the meningeal arteries, and is continuous with the periosteum on the outer surface of the skull at the foramen magnum and other intracranial foramina (Fig. 8.31B).
The inner meningeal layer is in close contact with the arachnoid mater and is continuous with the spinal dura mater through the foramen magnum.
The two layers of dura separate from each other at numerous locations to form two unique types of structures (Fig. 8.31A): dural partitions, which project inward and incompletely separate parts of the brain, and intracranial venous structures.
The dural partitions project into the cranial cavity and partially subdivide the cranial cavity. They include the falx cerebri, tentorium cerebelli, falx cerebelli, and diaphragma sellae.
The falx cerebri (Fig. 8.32) is a crescent-shaped downward projection of meningeal dura mater from the dura lining the calva that passes between the two cerebral hemispheres. It is attached anteriorly to the crista galli of the ethmoid bone and frontal crest of the frontal bone. Posteriorly it is attached to and blends with the tentorium cerebelli.