Bronchial tree casts were made from one or both lungs from nine children aged from five weeks to 17 years. The branches of the casts were ordered by the method of Strahler, and diameter ratio, length ratio, and branching ratio were determined. From five to 12 weeks the diameter ratio is about 1.35, but by 13 months it has risen to 1.45, indicating a faster rate of growth in the larger airways over the period; thereafter it stays at or above this level. The data show that the peripheral conducting airways are relatively large in diameter at 1-3 months, attaining their adult proportion after one year. In contrast, length ratio shows no trend with age. On the assumption of 25,000 terminal bronchioles, the mean number of orders between terminal bronchiole and trachea was estimated to be about 12.
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