Respiration and Metaholism 



221 



vided by forces of physical diffusion and docs not require the additional process 

 of secretion as Haldane '■''•' contended earlier.''^- "* Mice when administered 

 carbon dioxide containing the radioactive isotope C'^ show a rapid two-way 

 exchange of the gas across the lung surface with a total effective transfer from 

 the blood to the alveoli."''" 



In birds the characteristic lung mechanism equipped with air sacs permits a 

 two-way flushing of the lungs and provides for gas exchange during both 

 inspiration and expiration (Fig. 42). Calculations by Krogh-"' indicate that 



Fig. 42. Lungs and air sacs of pigeon photographed from a metal cast. 

 From Scharnke."" 



three fourths of the total inspiration (tidal volume) is taken up by air sacs 

 and one fourth by the lungs themselves. The respiratory exchange is auto- 

 maticallv increased during flight by the synchronization of wing and respira- 

 tory movements. Similar coordination between respiration and flight move- 

 ments occurs in bats.''^'' 



The lung tvpe of respiratory mechanism is advantageous not only by virtue 

 of size and of adaptabilit\- to homoiothermic activities, but also by reason of 

 the protection provided against mechanical damage and dehydration, and as 

 further opportunity for efficient xentilation. Lungs, probably derived from the 



