THE RESPIRATORY APPARATUS 205 



others. The bones that are always aerated in all birds are the 

 cervical and the dorsal vertebrae, the sternum, and the humeri. 

 Those aerated in some kinds only are the furculum, the scapulae^ 

 the vertebral and the sternal ribs, the sacrum, the coccyx, and the 

 femurs. Those that are never aerated are the bones of the forearm, 

 the hand, the leg, and the foot. 



The service of air to the bones in most parts of the body by the 

 air-sacs, as just shown, is in special cases otherwise rendered. The 

 Eustachian tubes furnish air to the bones of the cranium and to the 

 upper jaw; while the lower jaw receives air from the pneumatic 

 foramen situated upon each ramus behind the tympanic articulation, 

 and from an air cell which surrounds the joint. 



The cavities of the embryonic bones, which afterward become 

 pneumatic, are filled with marrow. Selenka states that the invasion 

 of the bones by the air is a late development, and that in the humerus 

 this invasion occurs after the twenty-second day in the life of the 

 chick. 



Hunter and Compar, who have made extensive researches, con- 

 sider the function of the air-sacs as threefold. 



First, the air-sacs are subsidiary respiratory organs, which aid 

 in ridding the blood of waste products and in taking in oxygen. 



Second, they aid mechanically the actions of respiration in birds. 

 During the act of inspiration the sternum is depressed, the angle 

 between the vertebral and the sternal ribs is made less acute, and 

 the thoracic cavity proportionately enlarged; the air then rushes 

 into the lungs and into the thoracic receptacles, while those 

 of the abdomen become flaccid. When the sternum is raised, or 

 approximated toward the spine, part of the air is expelled from the 

 lungs and the thoracic air-sac through the trachea, and part is 

 driven into the abdominal receptacles, which are thus alternately 

 enlarged and diminished with the expansion and the contraction of 

 the thorax. Hence the lungs, notwithstanding their fixed condition, 

 are subject to due compression through the medium of the contigu- 

 ous air receptacles, and are affected equally and regularly by every 

 motion of the sternum and of the ribs. 



Third, they reduce decidedly the specific gravity of the whole 

 body. This must necessarily follow from the large spaces filled with 

 air as well as from the absence in the bones of marrow and other 

 fluids. The air-sacs by their position also render equilibrium more 

 stable. 



