Light micrograph of a longitudinal section through a bundle (fascicle) of nerve fibres, or axons. Within each fascicle are many myelinated nerve fibres. Myelin (black) is an insulating fatty layer that surrounds single nerve fibres increasing the speed at which nerve impulses travel. Each fascicle is wrapped in a layer of connective tissue called the perineurium (brown). Magnification: x200 when printed at 15cm wide.