The terminal air sacs and their blood supply in a 37‐day infant lung

Wax models of a terminal spray of peripheral air passages from the middle lobe of a 37‐day infant lung are presented in three views. Superimposed on drawings of the models are reconstructions of the arteries and veins. The spray begins with a pre‐terminal bronchiole and ends in a trifurcation of alveolar ducts, each giving rise to two clusters of saccules. The third duct and its clusters are markedly under‐developed. So far there is no evidence that new acini are being formed and the possibility of this seems less and less likely. Terminal arteries do not follow the rigid pattern of Miller (1937), namely, that an artery accompanying an alveolar duct breaks up into as many branches as there are “atria” and each of these into as many branches as there are sacculi. Instead, the six clusters are partially supplied by two rami which branch irregularly and also supply adjacent clusters. Parts of a cluster may be supplied by arteries from distant and alien sources. A long arterial ramus frequently takes a short cut through saccules of other clusters. Conspicuous are numerous small adventitious branches arising from the main artery. Often a peripheral vein is continuous with a pleural vein.