Another big factor in mental decline has to do with myelin, that fatty coating around axons that serves as an insulator. White-matter tracts—the transmission lines of the brain, the myelin-coated axons—decay with age starting at age fifty or so, and remyelination slows down to the point where it can no longer keep up. While the gray matter of the human frontal lobe and hippocampus shrinks an average of about 14 percent between the ages of thirty and eighty, shrinkage of white matter is even more drastic, averaging 24 percent. Moreover, unlike gray matter, which shows a more gradual shrinkage
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