Quantitative histologic and teased‐fiber measurements were made on specimens of sural nerves of man in the first two decades of life, thus providing normal data not previously availabel. As previously reported for nerves from adults, there was a linear relationship between diameter and length of internodes: large fibers had long internodes and small fibers had short internodes. The slopes of the regression lines calculated for this relationship for each of these nerves became steeper with increasing age. In the second decade the slope became similar to that of nerves of adults. The transverse fascicular area increased particularly during the first five years of life. The number of myelinated fibers per nerve varied from approximately 4,000 to 12,300, the lowest value occurring in the nerve of the newborn. The frequency distribution of the myelinated fibers was unimodal in the newborn and was already bimodal by the seventh month. Whereas the peak of the small fiber group remained in the 2.0 to 2.9–μ size group in all nerves, the peak of the large fiber group shifted to larger size categories with increasing age, reaching adults values just after the age of five years. This correlates well with the age at which maximal conduction velocities are attained in the ulnar nerve. The diameter of the axis cylinder bore a linear relationship to the diameter of the whole myelinated fiber: small fibers had thin myelin sheaths and large fibers had thick ones. The value of the ratio of the diameters incresed progressively from small to large fibers, a result which is probably spurious. The slope of the regression lines for the relationship of diameter of axis cylinder to diameter of the whole fiber was the same for nerves of persons of different ages.
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