Arachnoid webs

Thin sheets of arachnoidA thin, transparent membrane, lying immediately over the brain and spinal cord, and in turn being surrounded by dura. The arachnoid forms thin membranes and strands likened by early anatomists to a spider’s web. Hence its name, referring to spiders, as in “arachnophobia”, which means the fear of spiders. tissueIn everyday speech a piece of tissue is a thin sheet of paper, used to wrap a present or, in a slightly different form, to blow the nose, or to be used in the toilet. In biology the word tissue refers to living material made up of cells or groups of cells of similar type or types. We may speak, for example, of nerve tissue or of fatty tissue, of glandular tissue or of connective tissue. Individual organs of the body, such as the brain, the heart, the liver or the kidneys, are made up of several tissue types, almost always including connective tissue., which form between the spinal cordThe main nerve trunk running down the spinal canal and connecting the individual’s mind and consciousness to the rest of his or her body. It is about the size of a person’s little finger, in diameter. It cannot repair itself if damaged. and its enclosing thecaThis Latin word means a sheath, or sleeve. It refers to the tube of dura that lines the spinal canal, retaining within it the spinal cerebrospinal fluid and containing the spinal cord and the nerves that issue from the cord., obstructing the normal flow of 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....