522 Comparative Morphology of Chordates 



outgrowths from the lungs. From each lung a single secondary bronchus 

 passes directly into each of four of the major adjacent sacs, the cervical, 

 interclavicular, anterior and posterior thoracic; but it is the meso- 

 bronchus which, passing to the posterior end of the lung, emerges to 

 connect with the great abdominal sac. In addition to the one large 

 air-tube entering the sac from the lung, each of the sacs except the 

 cervicals is connected with the adjacent region of the lung by several 

 comparatively small tubes, recurrent bronchi (Figs. 407, 410) which 

 open into the smaller bronchial tubes within the lungs. 



Smaller air-sacs, their number and distribution varying greatly 

 in different species, extend throughout the body of the bird. They may 

 occur among the viscera, beneath the skin, between muscles, and even 

 inside bones. The pneumaticity of the skeleton is a most extra- 

 ordinary feature of birds. In most birds the bones are, to greater or less 

 extent, occupied by air-filled spaces. In other animals the long bones 

 of the legs contain central cavities filled by a soft marrow. In birds the 

 central cavities of the long bones of leg and wing are usually filled 

 with air. Bones as remote from the lungs as the metatarsals may be 

 pneumatic. If the humerus of the wing of a dead pigeon is cut across, 

 a fluid (such as latex or hot melted wax) may be injected into the 

 central cavity of the bone and caused to flow into the interclavicular 

 sac and thence into the lungs, trachea, and at least the larger air- 

 sacs. In this way a cast of the main respiratory cavities may be ob- 

 tained. Not only the bones of the limbs but also the girdles, sternum, 



Fig. 410. Diagram of respiratory organs of a bird, left side view, (ab) Abdominal 

 air-sac; (ath) anterior thoracic air-sac; (c) cervical air-sac; (ht) heart; {id) inter- 

 clavicular air-sac; (Ig) left lung; (mbr) mesobronchus; {pho) posthepatic septum; 

 (pth) posterior thoracic air-sac; (rbr) recurrent bronchi; (tr) trachea. (From 

 Goodrich: "Studies on the Structure and Development of Vertebrates." By per- 

 mission of The Macrnillan Company, publishers.) 



