476 VERTEBRATE LIFE AND ORGANIZATION 



Glottis //"t \} r\\ . Larynx 



Anterior 



thoracic; 



Sac 



Lung 



Posterior — I 

 thoracic V 

 sac 



Abdlominal 

 sac 



^=^— CervicaL sac 



Interclavicalar sac 



Diverticula into 

 bones of the 

 pectoral girdle and 

 appendage 



Main 

 bronchus 



Parabronchi 

 and- ail" 

 capillaries 



Recurrent 

 bi'onchuS 



Figure 24.6. The respiratory organs of a bird as seen in a dorsal view. The course 

 of the main bronchus through the huig to the air sacs, the major branches of the main 

 bronchus, and the recurrent bronchi are shown on the right side. Minute parabronchi 

 and air capillaries, a few of which are shown, interconnect the recurrent bronchi and 

 the branches of the main bronchus. 



and softened by the uptake of water. Food is mixed with peptic enzymes 

 in the proventriculus, or first part of the stomach, and then passes into 

 the gizzard, the highly modified posterior part of the stomach charac- 

 terized by thick muscular walls and modified glands that secrete a horny 

 lining. Small stones that have been swallowed are usually found in the 

 gizzard and aid in grinding the food to a pulp and mixing it with 

 the gastric juices. The intestinal region is relatively short compared 

 to the intestine of mammals, and is lined with microscopic, finger-like 

 projections, the villi, that greatly increase the surface area. Digestion is 

 completed in this region with the aid of enzymes from the liver, pan- 

 creas and intestinal glands, and the digested food is absorbed. 



The anterior parts of the respiratory system are similar to those of 

 lower tetrapods, except that birds have a longer neck and hence a longer 

 windpipe, or trachea. The lungs themselves are relatively small and 

 compact organs, but they are subdivided internally into many passages 

 that greatly increase the respiratory surface. They are unusual in that 

 the two main bronchi which lead from the trachea not only communi- 

 cate ultimately with minute, vascularized air capillaries in the lung, 

 where gas exchange occurs, but also continue through the lung into a 



