402 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vot. 50 



ausgfiihrt werden." Among- these "Erwagungen" there is first of 

 all the assertion (p. 486) that in man every great effort of the 

 anterior extremity is accompanied by a more or less complete immo- 

 bility of the thorax. This is indeed the case in movements occupy- 

 ing only a short time, as in lifting a weight (one of the examples 

 mentioned by Baer), but not in movements of longer duration. A 

 trained gymnast breathes during the most difficult exercise if it lasts 

 long, only he so chooses the moment for making the respiratory 

 movements that he thereby suffers no loss of power. An untrained 

 gymnast, it is true, does not readily select such a moment, and there- 

 fore does not trust himself to breathe, and consequently is unable 

 to endure so long a muscular strain. Why should we not assume 

 that the flying bird has the ability of a good gymnast ? A flying sky- 

 lark by no means sings in the time of its wing-beats. These are 

 rapid — about eight in a second— so that one can hardly see the 

 moving wings. Nevertheless the sky-lark produces, during its 

 flight, long-drawn and quite continuous notes, which in their per- 

 fectly constant intensity, show no trace of being composed of numer- 

 ous short notes corresponding to the intervals between the wing- 

 beats. Such would be the case, however, if the view of Baer, given 

 above, which is shared by Campana and Strasser, were correct. But 

 there is also another consideration which makes that opinion un- 

 tenable. The only direct influence on the lungs and the air-sac 

 system of the wing-movement during flight is the alternation of 

 pressure exerted on the air-sac diverticula lying above and below 

 the shoulder-joint. Every time the wings are raised the former, 

 every time the wings are lowered the latter are compressed. As, 

 however, both are diverticula of one and the same sac, this alter- 

 nating compression of them caused by the wing-movement cannot 

 produce any considerable air-current passing through the lungs. 

 The air of the sac in question, the saccus interclavicularis, is merely 

 agitated to and fro by this means. In this connection it must not be 

 forgotten that, as Lendenfeld (1896, p. Jjj) has stated, the birds of 

 great wing-power which possess the largest axillary and subpectoral 

 diverticula are for the most part sailing fliers which often continue 

 their flight for many minutes, even for half an hour and longer 

 (Darwin, condor), without flapping their wings, so that in these 

 even that slight air-movement in the air-sac system is interrupted 

 for considerable spaces of time. 



We will now consider the question of the utility afforded by the 

 pneumaticity of the bones. It has been mentioned above that the 

 walls of the air-sacs within the bones are very rich in blood-vessels, 



