MtiTXEK] THE AIR-SACS 0? THE PIGEON 397 



schied zwischen der ausseren Atmosphaere und der Luft der Sacke 

 sich ganzlich ausgeglichen hat, beginnt die expiratorische Verengung 

 des Brustkorbes, ..." 



At the time of the expiratory contraction of the thorax there 

 arises, conversely, an excess of pressure in the intra-thoracic air- 

 sacs. The}' press against the lungs and compress them wherever 

 they are in contact with them. If the thorax of a living animal is 

 opened the assistance given by the varying pressure of the sacci 

 intermedii ceases and the changes of volume, which are then induced 

 by the muscles of Perrault only, are less than under normal condi- 

 tions. This circumstance may well be the principal reason for the 

 undue underrating of the respiratory lung movements by a great 

 number of recent authors. 



I will by no means deny that there occurs during respiration an 

 exchange of air between the air-sacs and the outer atmosphere. But 

 I fail to see how this can appreciably assist the pulmonary respiration. 

 The expansion and contraction of the lungs is synchronous with that 

 of the air-sacs. During inspiration all parts of these organs draw 

 air out of the bronchi. During expiration the air-sacs do indeed 

 eject some air into the bronchial branches, but this cannot reach the 

 parabronchia, because the latter, during respiration, likewise contract 

 and eject air. If the air thus ejected from the air-sacs does not pass 

 through the parabronchia, it cannot enter the alveoli which branch 

 off from them, and in which the blood oxidation takes place (cf. Sie- 

 fert, 1896, p. 476; and Lendenfeld, 1896, p. 776). The structure of 

 the lungs, which has been described above, precludes the possibility 

 of this air passing through the parabronchia by following the few 

 narrow communications that exist here and there. But even if it 

 were so, such an arrangement could hardly aid in the respiratory 

 function. Let us suppose that during inspiration the lungs and air- 

 sacs are filled with fresh air. The air which reaches the lungs would 

 there give off a portion of its oxygen ; that in the air-sacs would not. 

 During expiration the air from the lungs would be driven out 

 through the bronchi into the trachea, and the air from the sacs would 

 pass into the lungs. There the latter air would be deprived of a 

 portion of its oxygen. During the next inspiration this air (poor 

 in oxygen) would again pass from the lungs into the air-sacs. 

 During the next expiration the same air would then again fill the 

 lungs, and these would thus always contain air poor in oxygen, and 

 therefore ill adapted for respiratory purposes. 



There is also a second important circumstance in relation to this 

 subject that appears to have hitherto been overlooked. In birds we 



