The "white matter" of the central system consists chiefly of axons with their enveloping or medullary, sheath and neuroglia.
The neuroglia theory explains this very well and does away with the difficulty.
This may be applied to the neuroglia cells apparently as well as to the muscle fibers.
Ramon y Cajal has described and pictured a special kind of neuroglia cells for the gray and another for the white matter.
He has also described particularly a special form of neuroglia cells which lie close to the blood vessels.
neuroglia neu·rog·li·a (nu-rŏg'lē-ə, nyu-, nur'ə-glē'ə, -glī'-, nyur'-)
The delicate network of branched cells and fibers that supports the tissue of the central nervous system. Also called glia, reticulum.