Heretofore, the methods used for measuring the effect of various types of atelectasis on the pulmonary circulation have been too indirect to justify positive assurance as to the actual quantitative variations in blood volume that occur in a lung as a result of pneumothorax or bronchial obstruction. The evidence available is adequately reviewed in the recent exhaustive article by Coryllos and Birnbaum.1 In support of the ischemic effect of obstruction atelectasis, these authors offer further evidence by perfusion experiments of the lungs with india ink. Microscopic examination of the atelectatic lung, with the interalveolar capillaries rendered visible by this method, shows by comparison with the normal lung a marked decrease in the number of capillaries per unit of parenchyma. From this they conclude that the blood content per alveolar sac and, therefore, for the whole lung is less than for a normal lung. With Bruns,2 who used a