Sciatic nerve. Coloured scanning electron micrograph (SEM) of a section through a sciatic nerve, showing the nerve fibres (axons) surrounded by myelin sheaths (blue). Connective tissue fibres (thread-like) are also visible. The sciatic nerve is the longest and widest single nerve in the human body, running from the top of the leg to the back of the foot. Magnification: x1000 when printed at 10 centimetres wide.