Human Skull

Depending on their shape, bones are classified as long, short, flat or irregular. Bones of
different types contain different proportions of the two types of osseous tissue: compact and
spongy bone. While the former has a smooth structure, the latter is composed of small
needle-like or flat pieces of bone called trabeculae, which form a network filled with red or
yellow bone marrow. Most skull bones are flat and consist of two parallel compact bone
surfaces, with a layer of spongy bone sandwiched between. The spongy bone layer of flat
bones (the diploë) predominantly contains red bone marrow and hence has a high concen-
tration of blood.
The skull is a highly complex structure consisting of 22 bones altogether. These can be
divided into two sets, the cranial bones (or cranium) and the facial bones. While the latter
form the framework of the face, the cranial bones form the cranial cavity that encloses and
protects the brain. All bones of the adult skull are firmly connected by sutures. Figure 2–2
shows the most important bones of the skull. The  frontal bone forms the forehead and
contains the  frontal sinuses, which are air filled cells within the bone. Most superior and
lateral aspects of the skull are formed by the parietal bones while the occipital bone forms
the posterior aspects. The base of the occipital bone contains the  foramen magnum, which
is a large hole allowing the inferior part of the brain to connect to the spinal cord.
remaining bones of the cranium are the temporal, sphenoid and ethmoid bones.

Meninges

The meninges (Figure 2–3) are three connective tissue membranes enclosing the brain
the spinal cord. Their functions are to protect the CNS and blood vessels, enclos
venous sinuses, retain the  cerebrospinal fluid, and form partitions within the skull.
outermost meninx is the dura mater, which encloses the arachnoid mater and the inner
pia mater.

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