RESPIRATION IN AIR 



73 



slightly developed only, from the anterior air-sacs. They are 

 absent in penguins (Vos, 1 937). Provided a sufficient number 

 of microscopic anastomoses exist connecting the recurrent with 

 the main bronchi through the parenchyma, the flow of air 

 between the trachea and the sacs must pass to a corresponding 



Sclav 



Fig. 45. Diagram of pigeon lungs with air-sacs. Left, ventral surface; 

 right, dorsal surface with recurrent bronchi from abdominal and posterior 

 thoracic air-sac. (Brandes.) 



extent through the lung parenchyma. We have two connec- 

 tions in parallel, and the flow will be distributed in inverse 

 ratio of their resistance. 



The presence of valves forcing the air in definite directions 

 through the pulmonary tissue lias been postulated repeatedly 

 (Brandes, 1924; Vos, 1934). They have never been observed 

 anatomically, and the physiological experiments from which 

 their existence has been deduced are inconclusive. 



The volume of the lungs is quite small, but the air-sacs 

 take up a much larger proportion of the total body volume 



