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 Archeeopteryx 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. Ina 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 ccelom. 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- 
