RESPIRATION IN AIR 



71 



fly (Apteryx, Penguins) show these adaptations in a greatly 

 reduced form. 



The lungs of birds are small and compact. They adhere 

 firmly to the wall of the thorax, and a number of ribs are 

 embedded in their substance (Fig. 42). 



L.«K^i3E 



Fig. 43. Air-sacs of duck, inflated. (Sappey.) 



Certain large bronchi pass directly through the lungs and 

 open through wide "ostia" into spacious air-sacs (Figs. 43, 44, 

 45). It is convenient to distinguish between the anterior sacs 

 comprising the two cervical, the single clavicular, and the two 

 praethoracic, and the posterior comprising the paired post- 

 thoracic and abdominal. The air-sacs are very thin- walled 

 and have only a poor supply of arterial blood. No significant 

 exchange of gases takes place through their walls (Soum, 1 896). 

 The two main bronchi arising from the trachea lose their 

 cartilage shortly after entering the lungs and continue as 

 "mesobronchi" towards the posterior ends of the lungs, open- 

 ing there into the abdominal sacs. From the main- and 

 mesobronchi a definite small number of primary bronchi lead 

 to the other air-sacs and into the "parenchyma" of the lungs 

 (Fig. 45). The walls of all these bronchi and their ramifica- 

 tions are pierced like a sieve by an enormous number of holes 

 (about 1/2 mm wide) leading into "parabronchi." The walls 

 of these again are a meshwork and open into air-capillaries 



