Nerve fibres. Coloured light micrograph of a transverse section through three bundles (fascicles) of nerve fibres. Within each fascicle are many myelinated nerve fibres. Myelin (black) is an insulating fatty layer that surrounds single nerve fibres called axons (white/green circles), increasing the speed at which nerve impulses travel. Supporting the fibres within each fascicle is the perineurium (green), which is a form of connective tissue. All the fascicles within the nerve bundle are bound together by epineurium connective tissue (blue). Magnification: x200 when printed 10cm wide.