An investigation which involves injecting saline, slowly, into the cerebrospinal fluidThis clear, water-like fluid forms, for the most part, within the brain, more specifically inside the ventricles of the brain. Blood flows through structures inside the ventricles, called the choroid plexuses, and clear fluid is produced as a result, passing into the ventricle. This process is akin to blood flowing through the kidneys and urine being produced as a result. An important difference is that, unlike urine, the cerebrospinal fluid is not passed out of the body periodically. Instead, the fluid is re-absorbed back into the blood stream, mostly via the arachnoid granulations, into the cerebral venous sinuses. In the course of a 24-hour period, most healthy adults will produce about 500mls (or a pint) of cerebrospinal fluid, although at... pathways, either via a lumbar punctureA long needle is passed into the lower part of the spine, in order to remove some cerebrospinal fluid. This may be to reduce pressure within the head or to obtain a sample of the fluid for laboratory analysis. needle or into a reservoir connected to one of the brain ventriclesThe brain is not an entirely solid organ. There are chambers within the brain, referred to as ventricles or, more correctly, cerebral ventricles, to distinguish them from the ventricles of the heart. They represent the remnants of the central canal of the developing central nervous system, although they have taken on a much more complex shape than the original embryonic structure. There are four in total, one each side of the midline (referred to as the lateral ventricles) and two more, in the midline, referred to as the third and fourth ventricles, albeit often using Roman numerals.. The resulting rise in pressure is then measured, giving the investigator some idea as to how efficiently or otherwise the body is re-absorbing its own cerebrospinal fluidThis clear, water-like fluid forms, for the most part, within the brain, more specifically inside the ventricles of the brain. Blood flows through structures inside the ventricles, called the choroid plexuses, and clear fluid is produced as a result, passing into the ventricle. This process is akin to blood flowing through the kidneys and urine being produced as a result. An important difference is that, unlike urine, the cerebrospinal fluid is not passed out of the body periodically. Instead, the fluid is re-absorbed back into the blood stream, mostly via the arachnoid granulations, into the cerebral venous sinuses. In the course of a 24-hour period, most healthy adults will produce about 500mls (or a pint) of cerebrospinal fluid, although at....