The connective tissue septa in the adult human lung.

The septa of connective tissue which divide the lung have usually been studied because of their relation to secondary lobules or incidentally to investigation of some associated structure, such as lymphatics (Flint, 1906; Cunningham, 1916). In the human lung the secondary lobules are defined as the units of respiratory tissue which are enclosed within thin connective tissue septa (Laguesse and d'Hardivillier, 1897; Felix, 1928; Miller, 1947; Loosli and Potter, 1951; Birnbaum, 1954). Laguesse and d'Hardivillier described the septa as subtended from the larger lymphatics which form polyhedral rings on the pleura. Felix went further by comparing secondary lobules in different parts of the lung. He divided the lobe into cortex (Lappenmantel) and medulla (Lappenkern), the cortex being two lobules deep and separated from the medulla by strong connective tissue passing from medulla to pleura. Because of the detail of his description, in which he represented the septa diagrammatically as complete lines, his work is usually quoted whenever the anatomy of the peripheral part of the lung is discussed. This description of the septa implied that they are present throughout the lung and are complete, and it may be that Felix's technique of injection of the bronchial tree with subsequent corrosion of the lung was in part responsible for his assumption that the septa were fully developed. Fischer (1953) threw some doubt on this conclusion when he mentioned that the boundaries of the secondary lobules are usually not as clearly defined in the human lung as described by Felix. Later Van Allen and Lindskog (1930) showed that the lobules as described in this way by the anatomist are not self-contained units, since there is collateral drift of air between them. Their recognition of this phenomenon led to renewed study of the alveolar walls in the search for alveolar pores as the