The division of the mature mammalian brain and spinal cord into regions of "white" matter and "gray" matter is observable upon the most cursory inspection. Microscopic examination indicates that the "white" matter regions are characterized by a multilamellar sheath (myelin) which sur rounds the axons. A morphologically similar myelin imparts the white color to tracts of the peripheral nervous system, although, as is empha sized throughout the book, there are very significant morphological and chemical differences between central and peripheral myelin. The rapid postnatal accumulation of...