262 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



neck, and gives off a branch which forms an axillary sac in the axillary 

 region. Other sacs lie in the abdomen, lateral to the viscera, and are 

 called the anterior intermediate, posterior intermediate and 

 abdominal, the latter extending into the pelvis. From these air sacs 

 slender diverticula, not shown in the figures, extend among the viscera 

 and into certain of the bones. The pelvis, humerus, coracoid, sternum 

 and ribs most frequently contain prolongations of the air sacs are 

 pneumatic less frequently the femur, furcula and scapula. 



The functions of the air sacs are not certainly known. The fact that the walls 

 are supplied with blood by branches from the aorta negatives the idea that they are 

 respiratory. It has been suggested that they are concerned with the maintenance 

 of the equilibrium of the body during flight and that they also lessen the specific 

 gravity of the body. More plausible is the view that by the motion of the parts 

 about them they aid in the inspiration and expiration of air, especially during flight, 

 thus allowing the thoracic framework to remain rigid as an attachment of the 

 muscles, and at the same time causing the air to pass twice over the respiratory 

 surfaces of the lungs. The bones of the fossil bird Archeaopteryx were not pneu- 

 matic but those of some of the dinosaurian reptiles were. 



MAMMALS. The general structure of the mammalian lung was 

 outlined above (p. 256), The external shape is largely due to the 

 position in the pleural cavity, where it has to fit itself around the peri- 

 cardium, while it is flattened or truncate behind as a result of the 

 presence of the diaphragm. In a number of mammals (cetacea, sirenia, 

 horse, rhinoceros, Hyrax, etc.) both lungs are undivided, but usually 

 one or both are subdivided into lobes (the larger number in the right 

 lung), there being as many as five or six lobes in some species. In- 

 ternally there is a main bronchus from which dorsal and ventral 

 secondary bronchi arise, the ventral being the stronger. The bronchi 

 are supported and kept open by cartilages, rings in the larger, scattered 

 pieces in the smaller trunks. Frequently the bronchi are grouped as 

 eparterial and hyparterial (fig. 264), accordingly as they lie above 

 or below the pulmonary artery, but the distinction has little morpho- 

 logical value. Eparterial bronchi may be lacking or there may be 

 one or two in each lung. 



The phylogenetic history of the lungs is uncertain, one view being that they 

 have arisen from the air bladder of the fishes, the other being that they are modified 

 gill pouches, which, instead of growing laterally and fusing with the ectoderm, 

 have extended caudally and have encroached upon the coelom. In favor of the 

 former view are the double condition of the bladder in some ganoids, with alveolar 

 walls like those of the lungs of higher vertebrates, and the peculiarities of the pneu- 



