398 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOI,. SO 



find the trachea relatively much larger and the lungs relatively much 

 smaller, richer in capillaries, and heavier than in mammals. Conse- 

 quently the volume of air contained in the lungs of birds is relatively 

 much smaller than the volume of air contained in the lungs of mam- 

 mals. Even during the most powerful expiration only a part of this 

 small allowance of air is renewed by respiration, a considerable por- 

 tion remaining in the lungs as "residual" air. The amount of air 

 that is really expelled from the lungs would hardly suffice to fill the 

 very spacious trachea, on which account a large portion of the air 

 during respiration would only flow backward and forward between 

 the lungs and trachea, if this were not prevented by special arrange- 

 ments. Without such special arrangements oxygen would be sup- 

 plied to the respired air only by the slight mixing of tracheal with 

 external air at the nostrils, and a proper oxidation of the blood would 

 be impossible. 



In consequence of the change in volume of the air-sacs during 

 respiration the body of air moved is much greater, and thus the 

 above-mentioned disadvantage to breathing, that would otherwise 

 result from the great extent of the trachea is averted. The air that 

 leaves the air-sacs during expiration passes the ectobronchial 

 branches, and, together with the air that comes from the alveoli of 

 the lungs, enters the trachea, and is in great part expelled through 

 the nostrils. An equally large volume is thereupon again inhaled. 

 This is then, together with the air remaining in the trachea, distrib- 

 uted among the air-sacs and the lungs. In this way, indirectly, and 

 in no other way, the air-sacs assist in respiration. 



The width of the trachea diminishes the air-friction, and thus 

 economizes the labor of breathing, which, in view of the considerable 

 length of that organ, is of considerable importance. The length of 

 the trachea is determined by the length of the neck, and the latter 

 affords advantages of quite another sort. The length of the neck in 

 the flying Sauropsida, selectively acquired for other purposes, was 

 one of the causes of the development of the air-sac system, through 

 which its disadvantages in respect to respiration were compensated. 



The extra-thoracic air-sacs occupy interstices between the muscles 

 and other parts of the locomotory apparatus. According to Strasser, 

 this is an advantage, because in consequence there is "geringere 

 Kraft an innerer Arbeit verloren" (1877, p. 205) ; and (Strasser, 

 1877, P- 206) there is a "Gewinn an Grosse des Bewegungshebels 

 und an Bewegungsleistung ohne entsprechende Vermehrung der 

 Muskulatur und der iibrigen Organsysteme ;" as well as (Strasser, 

 1877, p. 207) "eine Vermehrung der Leistung, durch eine Ver- 



