The bronchus and pulmonary artery in this lung type maintain a close relationship throughout. The pulmonary vein, however, without the limiting supportive tissue septa as in type 1,, follows a more direct path to the hilum and does not maintain this close relationship (figs. 8, 22). Another marked difference is noted here. The pulmonary artery, in addition to supplying the distal portion of the respiratory bronchiole, the alveolar duct, and the alveoli, continues on and directly supplies the thin pleura (fig. 8). The bronchial artery, except for a small number of short branches in the hilum, contributes none of the pleural blood supply. It does, as in type 1,, supply the hilar lymph nodes, the pulmonary artery, the pulmonary vein, the bronchi, and the bronchioles -- terminating in a common capillary bed with the pulmonary artery at the level of the respiratory bronchiole. No bronchial artery pulmonary artery anastomoses were noted in this group.

Lung type 3, (fig. 3) is to some degree a composite of types 1, and 2,. It is characterized by the presence of incompletely developed secondary lobules; well defined, but haphazardly arranged, interlobular septa and a thick, remarkably vascular pleura (fig. 9). The most distal airways are similar to those found in type 1,, being composed of numerous, apparently true terminal bronchioles and occasional, poorly developed respiratory bronchioles (figs. 14, 15). In this instance, because of incomplete septation, the secondary lobule does not constitute in itself what appears to be a small individual lung as in type 1,. Air drifts from one area to another are, therefore, conceivable. Distally the bronchus is situated between a pulmonary artery on one side and a pulmonary vein on the other, as in type 1, (fig. 24). This relationship, however, is not maintained centrally. Here the pulmonary vein, as in type 2,, is noted to draw away from the bronchus, and to follow a more direct, independent course to the hilum (figs. 23, 24). The bronchial artery in its course and distribution differs somewhat from that found in other mammals. As seen in types 1, and 2,, it supplies the hilar lymph nodes, vasa vasorum to the pulmonary artery and vein, the bronchi and the terminal bronchioles. As in type 1,, it provides arterial blood to the interlobular septa, and an extremely rich anastomotic pleural supply is seen (figs. 9, 10). This pleural supply is derived both from hilar and interlobular bronchial artery branches. Such a dual derivation was strikingly demonstrated during the injection process where initial filling would be noted to occur in several isolated pleural vessels at once. Some of these were obviously filling from interlobular branches of the bronchial arteries while others were filling from direct hilar branches following along the pleural surface. With completion of filling, net-like anastomoses were noted to be present between these separately derived branches. An unusual increase in the number of bronchial arteries present within the substance of the lung was noted. This was accounted for primarily by the presence of a bronchial artery closely following the pulmonary artery. The diameter of this bronchial artery was much too large for it to be a mere vasa vasorum (figs. 16, 23, 24). In distal regions its diameter would be one-fourth to one-fifth that of the pulmonary artery. This vessel could be followed to the parenchyma where it directly provided bronchial arterial blood to the alveolar capillary bed (figs. 17, 18). Also three other direct pathways of alveolar bronchial arterial supply were noted: via the pleura; through the interlobular septa; and along the terminal bronchiole (figs. 14, 17, 18, 19). One bronchial arteriolar pulmonary arteriolar anastomosis was noted at the terminal bronchiolar level (fig. 26).