TEM of myelin sheath segment from transverse section of frog sciatic nerve. Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell. The main image shows a concentric array of dense and intermediate layers. Particulate sub-unit structures (see arrows) are regularly found within the plane of the layers in these well-preserved osmium fixed, low-temperature preparations. Magnification 480,000x @ 9x11 inches. Image B (lower right corner) is an cristae. A crista is a fold in the inner membrane of a mitochondrion. Magnification 600,000x. Image C (upper left corner) is a low angled x-ray diffraction pattern of rat sciatic nerve featuring a fundamental period of 178A with typical alternation of the intensities of the even and odd orders.