In normal lungs the bronchial arteries communicate with the pulmonary arteries through the capillary bed surrounding the respiratory bronchioles (Miller, 1947). Whether precapillary communications exist in normal lungs is open to question, but their development under abnormal circumstances is generally agreed. Liebow, Hales, and Bloomer (1959) have summarized the two main circumstances under which they have been found:(a) obstruction and diminished flow in the pulmonary arteries, and (b) formation of new tissue in the lung.

Many different anatomical forms of systemic-pulmonary anastomoses can be found in the lung, but hitherto only a few of these have been described in detail owing to the limitations of the techniques used. Cast preparations have been of great value in identifying the larger communica-tions, but this method has two disadvantages. Firstly, histological verification of anastomoses is impossible, and close apposition of minute vessels may beindistinguishable from their actual continuity. Secondly, the histological study of vessel wall structure and the surrounding tissue is denied. Angiographic studies of whole lungs have been of limited value because systemicpulmonary anastomoses cannot be distinguished from superimposition of non-communicating vessels.