THE BRAIN
Coverings of the brain
The Scalp
The scalp is made up of five distinct layers; the skin, the superficial fascia (a layer of subcutaneous fatty tissue), the galea aponeurotica, a layer of loose areolar tissue and the pericranium. The scalp has a very rich blood supply which enables it to heal very quickly if damaged (and accounts for the dramatic amounts of blood which can issue from relatively small scalp wounds).
The Cranium
The brain's first line of defence is the cranium, which together with the facial bones forms the skull. The cranium is made up of interconnecting plates, whose position and structure closely mirror the structures of the brain beneath.
The Meninges
Beneath the skull are three layers of covering known collectively as the meninges. From the outside in, they are: the dura mater, the arachnoid and the pia mater.
The dura mater (literally, "hard mother") is made up of two tough bluish membranes. The outer layer is the periosteum. The inner layer, the dura, lines the entire inside of the skull and creates folds in which parts of the brain are protected. One of these folds, the falx, separates the right and left half of the brain. The tentorium separates the cerebral hemispheres from the midbrain, brainstem and cerebellum.
Beneath the dura is the arachnoid. This thin membrane covers the entire brain and is made up of delicate, elastic tissue and blood vessels. The space between the dura and the arachnoid is known as the subdural space.
The membrane closest to the surface of the brain is called the pia mater (literally, "sweet mother"). It covers the whole surface of the brain, following its folds, and contains many blood vessels. The space between the arachnoid and pia is the subarachnoid space, through which cerebrospinal fluid flows.
Cerebrospinal Fluid
As well as the solid protection of the meninges and the skull, the brain is cushioned by being surrounded by and filled with a watery fluid known as cerebrospinal fluid (CSF). CSF is manufactured from blood by a special membrane known as choroid plexus within the four ventricles of the brain and flows through the ventricular system to the the spine and subarachnoid space, providing essential nutrients and carrying away waste products. When it reaches structures called arachnoid villi on the surface of the brain, it is re-absorbed into the blood stream.
Hydrocephalus
Sometimes the continuous, balanced process of production and absorption of CSF is disturbed, causing a condition known as hydrocephalus. This can occur for three main reasons:
- Overproduction of CSF (caused by choroid plexus papillomas)
- A blockage in the complex "plumbing" of the ventricular system (obstructive hydrocephalus)
- Problems with absorption of CSF (communicating hydrocephalus)
Hydrocephalus is often treated by the surgical insertion of a device known as a shunt, which diverts CSF into another part of the body, usually the abdomen.
The brain is made up of a number of different parts of varying sizes. Each part has its own individual properties, but only when all the areas work together does the brain function effectively.
