A prodigious amount of research has clarified many of the problems of lung structure at both gross and microscopic levels. The structural components of the larger elements of the respiratory tree are adequately described in current textbooks of histology; there is little disagreement concerning them. However, as the respiratory areas of the lung are approached, structures become increasingly delicate and their delineation becomes correspondingly more difficult. The point of greatest contention and the one most difficult to settle has been the question of whether or not the pulmonary alveolus is lined by a continuous epithelium. During the past century, hundreds of authors have applied a multitude of technical procedures in attempts to solve this problem and have arrived at conflicting conclusions as to the nature of the alveolar surface.
Since an understanding of normal and abnormal pulmonary physiology and a rational approach to the treatment of lung diseases must have