RESPIRATION IN AIR 



75 



that the 2 percentage, rises rapidly in the posterior air-sacs 

 and much more slowly in the anterior sacs. This means a 

 larger ventilation in relation to capacity of the posterior sacs. 

 Assuming, according to the measurements, a total volume per 

 breath of 35 ml and a "dead space" (volume of air passages) 

 of 4 ml, E. Zeuthen, in a paper to be published in the near 

 future, has utilized Vos's results to calculate the ventilation of 

 the single sacs. He finds 17 ml per breath for the abdominal 

 sacs (12% of the volume), 8 for the postthoracic (14%), 

 1 only for the praethoracic (4%) and an insignificant vol- 

 ume (less than 1%) for the clavicular. This would leave 

 35 — (17 + 8 + 1) = 9 ml for the volume entering the lungs 

 during inspiration, but it must be admitted that this latter 

 figure is very uncertain and the whole calculation should not 

 be taken too seriously. Perhaps it is safe to say that at least 

 3/4 of the total inspiration ultimately goes to the air-sacs 

 and at most 1/4 to the lungs. From the anatomy one would 

 expect the dilatation of the lungs to be definitely smaller. 

 Experiments have been planned to obtain more reliable 

 figures by means of indifferent gases. 



Several series of determinations have been made of the com- 

 position of the gas mixtures in the air-sacs of birds breathing 

 normally. The most reliable are those made by Vos (1935) 

 on the duck by introducing narrow cannulas into the sacs and 

 drawing out small samples when the bird was absolutely quiet. 

 These give for C0 2 



Table 7 



The well-ventilated posterior sacs should show a much lower 

 C0 2 percentage if the air entered them only through the 

 bronchi, and conversely the C0 2 percentage of the expired air 



