The brain, as well as the spinal cord, is surrounded by three layers of membranes: a tough outer layer (dura mater); a delicate, middle layer (arachnoid mater); and an inner layer, firmly attached to the surface of the brain (pia mater). Figure 139-1 illustrates these relationships. The cranial meninges are continuous with, and similar to, the spinal meninges through the foramen magnum. However, there is one important distinction: The cranial dura mater consists of two layers, and only one of these is continuous through the foramen magnum.
Coronal section through brain and meninges. (Reproduced with permission from Waxman SG. Clinical Neuroanatomy, 27th ed. NY: McGraw-Hill, 2013.)
This outermost layer consists of an outer periosteal layer and an inner meningeal layer. The outer periosteal layer is firmly attached to the skull, is the periosteum of the cranial cavity, and contains the meningeal arteries. This layer is continuous with the periosteum on the outer surface of the skull at the foramen magnum. The inner meningeal layer is in close contact with the arachnoid mater and is continuous with the spinal dura matter through the foramen magnum.
There are four dural partitions that project inward and incompletely separate parts of the brain:
Falx cerebri—The falx cerebri is a crescent-shaped projection of meningeal dura mater 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.
Falx cerebelli—The falx cerebelli is a small midline projection of meningeal dura mater in the posterior cranial fossa. It is attached posteriorly to the internal occipital crest.
Tentorium cerebelli—This layer is a projection of the dura mater that covers and separates the cerebellum in the posterior cranial fossa from the posterior parts of the cerebral hemispheres.
Diaphragma sellae—This small shelf of the meningeal dura mater covers the hypophysial fossa in the sella turcica of the sphenoid bone. There is an opening in the center of the diaphragm sellae through which passes the infundibulum, connecting the pituitary gland with the base of the brain.
The arterial supply to the dura mater travels in the outer periosteal layer of the dura. It consists of:
Anterior meningeal arteries. Located in the anterior cranial fossa, the anterior meningeal arteries are branches of the ethmoidal arteries.
Middle and accessory meningeal arteries. Located in the middle cranial fossa, the middle meningeal artery is a branch of the maxillary artery. It enters the middle cranial fossa through the foramen spinosum and divides into anterior and posterior branches.
Posterior meningeal artery and other meningeal branches. Located in the posterior cranial fossa, these arteries come from several sources.