Axones nerviosos - A cross section of a bundle of nerve fibers. Axons (orange) are wrapped in myelin (purple); an extension of the membrane of glial cells. Myelin has high lipid content, and acts to insulate the axons, causing them to transmit nerve impulses much faster.
Nerve bundle. Coloured scanning electron micrograph (SEM) of a freeze-fractured section through a bundle of myelinated nerve fibres. Myelin sheaths (yellow) can be seen surrounding the axons (blue). Perineurium (connective tissue, pink) surrounds the nerve bundle while endoneurium divides the individual fibres.
Lung tissue, coloured scanning electron micrograph (SEM). At upper centre is a capillary filled with red blood cells (erythrocytes) and surrounding it are alveolar ducts. Alveolar ducts are the narrowed endings of bronchioles that open into clusters of alveoli (air sacs, not seen). Magnification: x555 when printed 10 centimetres high. Credit: STEFAN DILLER/SCIENCE PHOTO LIBRARY