ABSTRACT
Casts of the pulmonary airways were originally used to obtain qualitative data on the organization of lung structures and their dimensions. With the advent of chest surgery, detailed knowledge of the bronchopulmonary segments of the lung became necessary. The study of lung casts made an important contribution in this field, especially from the work of Brock and Boyden. A complete cast of the conducting airways of the human lung consists of about 50,000 branches. The first step in reducing the data set to manageable proportions is to group elements of tree by some form of classification. There are two broad ways of doing this, one based on classifying branches and the other on classifying vertices. It is sometimes mathematically more convenient for the numerical value of orders to increase down the tree, as it does in the case of generations. This can be achieved by the use of inverse orders, which simply correspond to Horsfield orders with the numbering inverted.
