74 



RESPIRATORY MECHANISMS 



than the lungs in mammals. Vos (1935) determined the 

 volume of the air-sacs in the inspiratory position (less the 

 cervicals) in a duck of 1.4 kg to 280 ml or 20% of the whole 

 body (Table 7, p. 75). 



By the respiratory movements in the normal standing posi- 

 tion of a bird the whole thoracic and abdominal cavity is 

 enlarged in inspiration and air is sucked into the lungs and 

 air-sacs. This was first clearly recognized by Soum (1896) 

 and a recent detailed study was made by Zimmer (1935). 

 The movements are shown in figs. 46 and 47. 



Fig. 46 



Fig. 47 



Fig. 46. Lateral view of respiratory thorax movements in standing bird- 

 (Zimmer.) 



Fig. 47. Frontal view of respiratory movements. Dotted line inspiration. 

 (Zimmer.) 



A study of the distribution of the inspired air between the 

 lungs and the separate air-sacs presents very formidable ex- 

 perimental difficulties, and from the literature anything but 

 a clear and consistent picture can be obtained. Some in- 

 formation is available, however. 



The lungs are expanded in inspiration mainly by the pull 

 of the ribs, as observed by Scharnke (1938) and others, but 

 just how much it has been impossible so far to find out. A 

 pigeon on which all the air-sacs have been broadly opened 

 gets very dyspnoeic, but is not asphyxiated. 



Experiment by Vos (1935), in which a duck was suddenly 

 given pure oxygen to breathe and the 2 percentage in the 

 single air-sacs measured after a few respirations, go to show 



