The Self-Mending Net
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the fibers of the peripheral nerves, are surrounded by Schwann cells.
These form tubes, made up of spiraling layers of membrane rich in a
fatty substance called myelin, around some of the largest fibers. A third
type, ependymal cells, line the four cavities within the brain, or ventri-
cles, and the narrow central canal that runs the length of the spinal cord.
These cells are close relatives, having all developed from the same part of
the ectoderm, or outer cell layer, that formed the primitive neural tube
in the embryo. The nervous system actually consists of several times
SCHWANN CELL SHEATH AROUND A PERIPHERAL NERVE FIBER
Until recently the perineural cells were considered merely a "packing
tissue," whose only job was to insulate and support the neurons. We've
now learned that they play a major role in getting nutrients to the neu-
rons. They also help control the diffusion of ions through the nerve cell
membrane and hence regulate the speed of impulse firing, even to the