RESPIRATION IN AIR 77 



the regulation of heat loss becomes a very important con- 

 sideration during flight. 



The respiration during flight. When the study of the respira- 

 tion in a resting bird presents serious difficulties this is doubly 

 true when we have to deal with the respiration during flight. 

 Until recently it was generally believed, mainly on the author- 

 ity of Baer (1896), that during flight the thorax is absolutely 

 fixed, while the ventilation is brought about by accessory sacs, 

 mainly those diverticula of the clavicular sac which separate 

 the wing muscles and may penetrate into them. These sacs 

 are alternately compressed and dilated by the muscle contrac- 

 tions. This conception does not take into account the enor- 

 mous increase in metabolism during active flight which 

 necessitates, like other muscular work, a corresponding in- 

 crease in pulmonary ventilation. Baer made the observation 

 (repeatedly verified) that a rapid stream of air against the 

 head of a bird inhibits the normal respiration (for a short time) 

 and took this as a proof of his contention. Marey (1890) 

 and Soum (1896) assumed a ventilation during flight by 

 movements of the thorax caused directly by the beating of 

 the wings. Soum showed that artificial wing movements 

 on a dead bird brought about ventilation, and Marey defi- 

 nitely showed by graphic recording that in a pigeon the venti- 

 lation during flight is synchronous with the wing movements. 

 Both Soum and Marey took it for granted that the down-stroke 

 of the wings corresponds to an expiration, and it must be 

 admitted that this is a priori much more probable than the 

 opposite. Nevertheless the most recent study by Zimmer 

 (1935) purports to show that the respiration during flight is in 

 principle independent of the flight movements. It is, accord- 

 ing to his graphic records, almost universally synchronized 

 with these, but expiration corresponds to the up-stroke of the 

 wings. I cannot accept Zimmer's records as quite conclusive 

 and would prefer to leave the question open for the present. 

 Zeuthen attempted through aeronautic considerations to 

 obtain some idea of the work performed by a bird during flight 



