The structure of the pulmonary acinus in a child of six years and eight months

So far as known this is the first portrayal of the detailed structure of a pulmonary acinus as seen both from within its cavities and from without. Some 200 serial sections (× 50) were traced upon transparent plastic plates and then viewed as translucent objects in packets of ten. This method was supplemented by wax‐plate and graphic reconstructions of the whole and of its parts. The pulmonary acinus is a terminal bronchiole and all its branches. In this example there were three generations of respiratory bronchioles and from two to five generations of alveolar ducts ending in saccules. This acinus had a volume of 15.6 mm3, comparable to a cube measuring 2.5 mm on each side. The two halves (medial and lateral semiacini) interdigitated. Only the lateral, forming 53.4% of the whole, was analyzed in detail. It consisted of a roof having three portions with varying patterns (namely a proximal wing, a distal wing extending to the connective tissue septum, and a rudimentary lateral component); a septal portion resting on the septum and consisting of four pairs of alternating small and large clusters of alveolar ducts, to the lower half of which a cluster of vesicles was appended; and a paired lateral envelope of ducts which in one place communicated with an adjacent acinus, thereby revealing a third mode of collateral ventilation. This acinus, reconstructed over a period of three years, is replete with numerous variations such as dilated atria and saccules, supernumerary structures, recurrent ducts, irregular branches and differing lengths of airways. Thus, it records the “fight for space” in earlier periods of growth. Knowing the pattern of this particular acinus, it has been possible to calculate the rate of diffusion of gases between the terminal bronchiole and the peripheral saccules, and vice versa. Yet the impression persists that no two acini are alike in either proximal or distal portions; therefore, it is questionable whether diffusion of gases in the peripheral airways of the living individual are really subject to precise mathematical measurement.