It is evident that many marked and striking differences exist between lungs when an inter-species comparison is made. The significance of these differences has not been studied nor has the existence of corresponding physiologic differences been determined. However, the dynamics of airflow, from morphologic considerations alone, may conceivably be different in the monkey than in the horse. The volume and, perhaps, even the characteristics of bronchial arterial blood flow might be different in the dog than in the horse. Also, interlobular air drifts may be all but nonexistent in the cow; probably occur in the horse much as in the human being; and, in contrast are present to a relatively immense degree on a segmental basis in the dog where lobules are absent (Van Allen and Lindskog,' 31). A reason for such wide variation in the pulmonary morphology is entirely lacking at present.

Within certain wide limits anatomy dictates function and, if one is permitted to speculate, potential pathology should be included in this statement as well. For example, the marked susceptibility of the monkey to respiratory infection might be related to its delicate, long alveolar ducts and short, large bronchioles situated within a parenchyma entirely lacking in protective supportive tissue barriers such as those found in types 1, and 3,. One might also wonder if monkeys are capable of developing bronchiolitis as we know it in man or the horse. In addition, it would be difficult to imagine chronic generalized emphysema occurring in a cow, considering its marked lobular development but, conversely, not difficult to imagine this occurring in the horse or the dog.