NERVOUS SYSTEM: CEREBROSPINAL FLUID
All these nerves are protected by sheaths of tougher tissue. The fibers of the central nervous system are left bare, but man's brain and spinal cord are surrounded by three layers of membranes, called meninges. They are possibly little known to you except as they become infected, thus producing meningitis. In between these membranes is the cerebro-spinal fluid, which allows of a little motion and cushions shocks to the delicate nervous tissue. The fluid not only surrounds the brain and cord but fills some spaces inside the brain. Within each side of the front part of the brain is a space lined with many fine blood vessels, and most of the fluid is formed there.
From these lateral ventricles, as they are called, the fluid passes into a central chamber called the third ventricle. From here is a little passage whose name I love - iter a tertio ad quartum ventriculum-the canal from the third to the fourth ventricle. Also called the aqueduct of Sylvius. I enjoy flattering myself that liking these old historical names is in the nature of preferring a Goddard desk to a Grand Rapids table.
From this ventricle the fluid goes through a lot of little holes to the spinal cord and the outer surfaces of the brain and cord, surrounding them. Afterwards it is absorbed into some big sinuses or veins around the brain. You see that it is a sort of little circulation in itself.
The fluid is always under pressure. When physicians want information about it, they put a needle between two of the lumber vertebrae; that is, in the lower back. Sometimes for unusual conditions they make a cistern puncture at the back of the neck just below the skull. This latter is a much more tricky procedure as it is near the medulla oblongata, the very delicate place where the brain and cord join. But sometimes a puncture here is of great value.
As the fluid runs out through the needle, its pressure can be measured. Low pressure is not particularly important but high pressure may be. Inflammation or tumors may make great pressure as may injuries with resulting bleeding within the skull. Certain drugs are said to increase pressure. Morphia is one of these, which is unfortunate as it may lead to patients' being refused morphia when they are suffering great pain.
The color of the fluid may give valuable information. Ordinarily it is clear and colorless, like water; in certain conditions, as some types of meningitis, it is turbid or cloudy. For years neurologists impressed me greatly when they spoke of the gold curve, meaningless to me, and especially with their references to xanthochromic fluid. I have always stood in awe of these learned men, but a few years ago I looked up xanthochromic and found that its full meaning is just yellow-colored, expressed in Greek. Words like this are a harmless hobby of neurologists and psychiatrists, who really know a lot of things that you could not understand in any language.
So let us hope that you never have to get concerned about your spinal fluid. If the examination proves "interesting" to the physicians concerned, that probably means trouble to you. With you, as with the little Eton boys, sporting on the banks of the Thames: "Where ignorance is bliss, 'tis folly to be wise."
Nowadays a goodly proportion of you have had your attention called to the spinal fluid by the familiar spinal anesthesia.
Once again a lumber puncture is used for that is the easiest place to introduce a needle. A few drops of fluid are first removed and the drug which produces the anesthesia is dissolved in this and introduced into the spinal canal. There are lots of little tricks to have the anesthesia begin at the desired level, to make it last longer or shorter times, etc. Both feeling and ability to use the muscles are stopped in this way.
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