RESPIRATORY ORGANS- 



31 



phibia. These bronchioles may be as large as, or even larger 

 than, the smaller bronchi ; but they differ from them in the ab- 

 sence of cartilage and glands in the wall, in the absence (usually) 

 of cilia on the internal surface, and in the existence of alveoli 

 arising directly from their walls. Besides the lobulation implied 

 by this branching of the respira- 

 tory ducts, the lungs may also 

 be divided into lobes, varying 

 in number, clearly recognizable 

 from the exterior. 



The lungs of birds are pe- 

 culiar in several respects. The 

 primary bronchus, after entering 

 the lung, continues along the 

 ventral surface to near the end 

 of the organ. Near its entrance 

 it gives off several lateral bron- 

 chi, which also course along the 

 ventral surface, and extend onto 

 its sides. The primary bron- 

 chus also gives off from its dor- 

 sal surface a larger number of 

 secondary bronchi, which extend 

 through to the dorsal surface. 

 From these dorsal and lateral 

 bronchi arise numbers of slen- 

 der tubes, the so-called lung- 

 pipes or parabronchi, which are to be regarded as modified bron- 

 chioles, since they have similar walls, and since they connect 

 with the infundibula. They differ, however, from the bron- 

 chioles of the mammals in that they unite or anastomose fre- 

 quently with each other, thus converting the whole lung into a 

 network of tubes (compare the condition in chameleon, above). 



In the embryonic avian lung thin-walled sacs arise from the 

 outer surface of the lung. These air sacs increase in size, and ex- 

 tend themselves in every direction, — into the abdominal cavity, 

 where they enter between the viscera, in between the muscles, 

 and beneath the skin ; they enter many of the bones (especially 



Fig. 34. Diagram of lung struc- 

 ture in man. B, bronchi; BL, 

 bronchioles ; A, alveolar duct ; /, 

 infundibulum, surrounded by alveoli. 

 Only a very few bronchi shown. 



