Anatomically, the horse lung appears to be remarkably like that of man, insofar as this can be ascertained from comparison of our findings in the horse with those of others (Birnbaum,' 54) in the human being. The only area in which one might find major disagreement in this matter is in regard to the alveolar distribution of the bronchial arteries. As early as 1858, Le Fort claimed an alveolar distribution of the bronchial arteries in human beings. In 1951, this was reaffirmed by Cudkowicz. The opposition to this point of view has its staunchest support in the work of Miller (' 50). Apparently, however, Miller has relied heavily on the anatomy in dogs and cats, and he has been criticized for using pathologic human material in his normal study (Loosli,' 38). Although Miller noted in 1907 that a difference in the pleural blood supply existed between animals, nowhere in his published works is it found that he did a comparative study of the intrapulmonary features of various mammalian lungs other than in the dog and cat (Miller,' 13;' 25).

The meaning of this variation in distribution of the bronchial artery as found in the horse is not clear. However, this artery is known to be a nutrient vessel with a distribution primarily to the proximal airways and supportive tissues of the lung. The alveoli and respiratory bronchioles are primarily diffusing tissues. Theoretically, they are capable of extracting their required oxygen either from the surrounding air (Ghoreyeb and Karsner,' 13) or from pulmonary arterial blood (Comroe,' 58). Therefore, an explanation of this alveolar bronchial artery supply might be the nutritive requirement of an increased amount of supportive tissue, not primarily diffusing in nature, in the region of the alveolus. If this be true, the possibility exists that an occlusive lesion of the bronchial arteries might cause widespread degeneration of supportive tissue similar to that seen in generalized emphysema. One would not expect such an event to occur in animals possessing lungs of types 1, or 2,.