Peripheral nerve. Light micrograph of a cross section through a peripheral nerve fascicle. A fascicle contains a bundle of nerve cell axons (the fibres down which impulses are transmitted) and is surrounded by connective tissue called perineurium. The myelin sheaths around the axons of many of the nerve fibres are visible (brown circles). Variation in the sizes of the axons can be seen. This reflects the diversity of functions carried out by peripheral nerves.