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Coverings of the central nervous system
The central nervous system (CNS) is supported and protected by bone and membranous coverings. The brain is located within the cranial cavity of the skull and the spinal cord lies in the vertebral, or spinal, canal within the vertebral column, or spine. Within their bony coverings, the brain and spinal cord are invested by three concentric membranous envelopes. The outermost membrane is the dura mater, the middle layer is the arachnoid mater and the innermost layer is the pia mater. The vertebral column and spinal meninges are described in Chapter 8 ; consequently, only the skull and cranial meninges are considered here.
Skull
The brain lies on the floor of the cranial cavity, which, together with the bones of the cranial vault, provides support and protection from physical injury. The floor of the cranial cavity consists of three fossae . Each of these accommodates particular parts of the brain and possesses foramina through which cranial nerves and blood vessels enter and leave the cranial cavity ( Fig. 5.1 ).
Anterior cranial fossa
The anterior cranial fossa is formed by the frontal, ethmoid and sphenoid bones. It accommodates the frontal lobes of the brain. The greater part of the floor of the anterior cranial fossa consists of the frontal bone and it also forms the roof of the orbit. The part of the frontal bone that forms the anterior wall of the fossa contains the frontal air sinus . The medial part of the floor of the anterior cranial fossa is composed of the ethmoid bone. In the midline, a sharp ridge, the crista galli, is the anterior point of attachment of the dural falx cerebri. In a narrow, elongated depression on either side of the crista galli lies the cribriform plate of the ethmoid bone, upon which lies the olfactory bulb . The bone of the cribriform plate is peppered with small perforations through which the fascicles of the olfactory nerve enter the cranial cavity from the nasal cavity to attach to the olfactory bulb.
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