Nerve fibres. Light micrograph (LM) of a transverse section through a bundle (fascicle) of nerve fibres. Within each fascicle are many myelinated nerve fibres. Myelin (green) is an insulating fatty layer that surrounds single nerve fibres called axons increasing the speed at which nerve impulses travel. Supporting the fibres within each fascicle is the endoneurium, which is a form of connective tissue. The perineurium surrounds each fascicle, which are in turn bound together by epineurial connective tissue (cyan). Magnification: x80 when printed 10cm wide.