76 RESPIRATORY MECHANISMS 



should be much lower if the expired air was made up as indi- 

 cated by Zeuthen's calculations of 75% air from the posterior 

 sacs and 25% from the lungs. The inference appears un- 

 avoidable that both during inspiration and during expiration 

 a rather large proportion of the air going to and from the 

 sacs passes the parenchyma of the lungs through the para- 

 bronchial and alveolar anastomoses. As pointed out above, 

 this is what one would expect from the anatomical structure, 

 without having to assume the presence of any valves. Quan- 

 titatively the figures would indicate that in the resting duck 

 during inspiration rather less than 1 /2 of the air entering the 

 sacs passes through the lung parenchyma, and during expira- 

 tion rather more than 1/2 passes out along this route, but this 

 is a very rough estimate. It goes without saying that the 

 passage of air with a high oxygen percentage and low C0 2 

 through the lung tissue both during inspiration and during 

 expiration must greatly facilitate the gas exchange. 



It is important to remember that the parabronchi are well 

 supplied with smooth muscle. The resistance to the passage 

 of air through them can therefore probably be regulated so as to 

 change the ratio of ventilation through the parenchyma to 

 that going directly to the air-sacs. This is of importance for 

 the heat-regulating function of the respiratory system. The 

 birds are well protected against heat loss through the skin by 

 the highly insulating plumage. They have no sweat glands 

 and the evaporation from the skin must be of a low order and 

 can be regulated, if at all, only to a slight extent. No meas- 

 urements are available. Evaporation of water from the 

 respiratory tract is therefore in birds, as in certain mammals 

 (e.g., dogs), an important mechanism for heat dissipation and 

 is regulated accordingly. Von Saalfeld (1936) found on a 

 pigeon, which had at a normal body temperature (41.7°C) 

 a respiration frequency of 45/min. with a depth of 4 ml, that 

 by increasing the body temperature to 43.6° the frequency 

 would increase to 500 with a depth of 1 .25 ml only. This will 

 enormously increase the evaporation of water and may leave 

 the pulmonary ventilation proper unaltered. As we shall see, 



