As the cerebrum grows, intricate interconnecting cavities called ventricles develop. In the lateral ventricles, formed by the cerebral hemispheres, there is a folded membrane rich incapillaries. It is called the plexus choreoideus. This membrane secretes a fluid, called the cerebrospinal fluid. It is concerned with maintaining intracranial pressure, acts as a shock absorber and prevents the brain from drying out. The cerebrospinal fluid flows through the ventricles and emerges through three small holes to the surface of the brain and the spinal cord. It bathes these structures. Because it is constantly formed, it must be constantly absorbed into the stream again. If this delicate balance is disturbed a condition called high intracranial pressure develops. In an infant this gives rise to hydrocephalus. This can be succesfully treated by inserting a tube to drain excess fluid into a vein and so reduce the intracranial pressure. The brain is safely encased by the skull, and thus protected from direct damage. The brain is further covered by three membranes and cerebrospinal fluid circulates between the outer two and the inner third, acting as a shock absorber. These membranes are called meninges. The dura, the hard membrane, is the outermost rneninx, made of tough connective tissue. Its main function is mechanical protection. On the inside of the dura lies the arachnoid membrane. By projecting connective tissue strands inwards, it forms the subarachnoid space. Here the cerebrospinal fluid circulates, and is reabsorbed into the bloodstream by the arachnoidea, a delicate membrane that resembles a cobweb. Following every convolution of the brain, the innermost meninx, the pia (soft) membrane, is very rich in blood vessels. Small capillaries branch from it to the inner structures of the brain. The pia is largely concerned with supplying food and oxygen. Chemical substances in blood have to cross the soft membrane before they can reach the nerve of the brain. The advantage of such a barrier is that poisonous substances cannot influence the brain too quickly.
On the other hand drugs that have to act on the brain are similarly inhibited.
Sometimes, as a result of infection, the brain membranes become inflamed, resulting in encephalitis or meningitis. Encephalitis is usually caused by a virus. One form, also called sleepy sickness, reached epidemic proportions in 1917-1924 and killed thousands of young people. The virus is often passed through the bites of ticks or mosquitoes, although the disorder may occur as a rare complication of small pox vaccination or a childhood infection such as measles. Meningitis is generally caused by bacteria, including those that are also responsible for tuberculosis and pneumonia. There is also a viral form, and both may occur as epidemics. Treatment is with drugs.