One is a protective sheath that keeps cells alive for months outside a host body, nourished by a layer of fat.

FORBES: The Science of Small

In MS, the protective sheath or layer around nerves, called myelin, comes under attack which, in turn, leaves the nerves open to damage.

BBC: Simple eye scan can reveal extent of Multiple Sclerosis

With MS, the protective layer around nerves, known as the myelin sheath, becomes damaged.

BBC: Brain scan of MS patient