Experimental massive pulmonary collapse.

Interest in atelectasis has in recent years been revived by the increased recognition of its frequency. It has been demonstrated that complete obstruction of the air passage supplying a unit of the lung which does not also communicate with another air passage will produce atelectasis.'3 It has been shown4 that this is due to absorption by the body fluids of the gases contained in that part of the lung. The process of collapse of a lung following complete bronchial obstruction consumes a considerable amount of time, but in some cases of undoubted atelectasis the collapse occurs quite rapidly.5'#{176} For this reason, and because bronchial breath sounds may sometimes be heard over the dull area, It has been hypothesized that collapse of the lung may occur rapidly from causes other than compression of the lung or bronchial obstruction.7 What these causes are has not been explained. Jackson8 has suggested that rapidly occurring atelectasis is due to the presence of a one way valve in the bronchus, and states that he has seen bronchos-copically such valves in action. The objection has, however, been made that an obstruction would in every case cause absence of breath and voice sounds and diminished tactile fremitus over the affected area. The anatomy of the respiratory tract does not seem to fit in with the idea of active collapse, but rather with the conception of passive collapse. The muscle fibers are arranged In a geodesic network around the bronchi and bronchioles, terminating in sphincters at the entrances to the alveoli,9 and their contraction cannot collapse the lung. These muscle fibers do not surround the alveoli and are not present in the human pleura.1#{176} The elastic fibers, on the other hand, are arranged in the longitudinal axis of the bronchial tree, for the most part, and surround the individual alveoli. They are present In large numbers in the pleura.9 This arrangement is an ideal one for expelling air from the lung, and this is in fact its purpose, the expulsion of tidal air being largely accomplished by this tissue during normal respiration. Elastic tissue, of course, has no power of active contraction.