150 Prof. R. von Lendenfeld on the Physiological 



organs or an auxiliary breathing-apparatus; (2) they are 

 exchisively mechanical locomotor organs (structures for the 

 maintenance of equilibrium and the regulation of specific 

 gravity) ; (3) they perform both of these functions. 



We might well suppose that good and powerful fliers, just 

 as they have a relatively heavier heart than other birds 

 (Parrot), also need especially efficient respiratory organs, and 

 that accordingly their air-sacs are breathing-organs. The 

 air-sacs themselves, however, cannot be respiratory organs, on 

 the one hand since they exhibit no arrangements whatever 

 for increasing their surface (folds, cells, or similar develop- 

 ments), and on the other hand because (in birds) the membrane 

 which bounds them is in general poorly supplied with blood- 

 vessels (Drosier and other authors). It is only on the inner 

 surface of pneumatic bones that we find capillaries in greater 

 abundance, and here, moreover, the excretion of carbonic acid 

 has been demonstrated experimentally (Baer) . Nevertheless 

 the respiration in this case cannot be anything considerable, 

 since in these rigid cjeca-like chambers the air must be in a 

 state of almost complete stagnation. But although oxidation 

 of the blood takes place only to an altogether inconsiderable 

 extent in the walls of the air-sacs, these structures might 

 still as a motor-apparatus, as bellows, play an important part 

 in respiration ; they might have to provide for the active 

 ventilation of the parts which actually oxidize the blood. It 

 was formerly supposed that the intra- and extra-thoracic air- 

 sacs of birds contracted alternately, and that by this means 

 the air was forced to and fro through the lungs (Sappey). 

 The results of later investigations, however, obtained by the 

 aid of the graphic method, and as to the correctness of which 

 no doubt whatever can exist, have shown that the air-pressure 

 in all air-sacs rises and falls simultaneously. There is con- 

 sequently no current of air from air-sac to air-sac, but only 

 a current ])assing to and fro between the mouth and the air- 

 sacs. Now although the air-sacs communicate directly with 

 the bronchi through wide open tubes, nevertheless a great 

 part of tiie air that streams to and fro between the nioutli and 

 the air-sacs, especially in expiration, is said to pass through 

 the fine canals of the lungs and to renew continually the air 

 in the pulmonary alveoli (Baer). To this, however, it must 

 be objected that the anatomical conditions do not support the 

 justice of this assumption. If this were the sole or the most 

 essential function of the air-sacs, we should, at any rate, 

 expect to meet with arrangements which would cause the 

 whole of the air in the air-sacs, or, at least, the greater portion 

 of it, to pass through the actual respiratory part of the 



