Number of axons and myelin sheaths in adult human medullary pyramids

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Abstract
As estimated from silver impregnated sections from 21 adult human medullas, the mean number of axons in 1 medullary pyramid is 1,087,200. As estimated from 12 of these medullas, stained with iron hematoxylin, the mean number of myelin sheaths is 1,033,400, or 93.8% of the number of axons. As in all comparisons of this type, the close agreement of the axon and myelin-sheath totals raises the question of whether the difference is the result of a systematic or technical error in the census. The limitations of the study are those of light microscopy and staining methods as well as the problem of observer error. The observed range of medullary pyramidal axons in this series extends from 749,900 to 1,391,100, while the expected range, including 95% of the population having structurally normal brains, is from 739,700 to 1,434,800. The percentage of myelin sheaths extends from 80.4 to 104.1%. The excess of myelin sheaths over axons in 2 specimens is explained by the variability of the totals owing to random sampling error. The lower limit of the range for axons in man overlaps the range for the chimpanzee and the seal, but the range for man will not overlap the macaque monkey.