448 AVES. 



tomose. The tertiary bronchi give off the fine canaliculi through 

 the walls of which the respiratory exchanges take place. 



From the above account it becomes quite clear that the 

 mechanism of respiration of birds must be quite different from 

 that of mammals. In birds, when the thorax and abdomen 

 dilate, air rushes, not into the lungs, which being but slightly 

 distensible are but little affected, but right down the main bron- 

 chi into the air-sacs. These therefore act as reservoirs of air 

 which affects by diffusion the composition of the air in the peri- 

 pheral extensions of the air- sacs, and of the air in the ultimate 

 pulmonary passages, which latter must be almost entirely of the 

 nature of what physiologists call residual air. 



The rate of respiration in birds is considerably greater than in 

 mammals. But whereas in mammals the tidal air, i.e. the air 

 taken in and given out in each inspiration and expiration, is only 

 renewed once by each respiratory act, i.e. by inspiration, in birds it 

 is renewed twice : for the air in the bronchial passages is renewed 

 in inspiration from the outside, and again in expiration by the 

 unvitiated air of the air-sacs. It follows therefore that each 

 respiratory act is practically twice as effective in birds as it is in 

 mammals. It seems clear then that the respiratory mechanism 

 of birds is a much more efficient apparatus than that of a mammal 

 and that the oxidation of the blood must be much more com- 

 plete. This is what we should expect when we remember the 

 much greater activity of birds, and the wonderful way in which 

 they are able to combine intense muscular action with the most 

 remarkable production of voice. 



The exact means by which the thorax and abdomen are 

 dilated and compressed in the act of respiration are not fully 

 understood. But having regard to the fact that in a bird the 

 air-sacs are always found distended after death, it seems pro- 

 bable that under ordinary circumstances expiration is a mus- 

 cular action, by which the thorax and abdomen are diminished 

 in size, and that inspiration is a purely passive action due to the 

 rebound of the thoracic framework and abdominal wall, in virtue 

 of their elasticity, after the muscular tension is relaxed. Whether 

 the backbone is raised or the sternum lowered in this rebound 

 no doubt depends upon what the bird is doing. If it is standing, 

 the sternum will be lowered, while on the other hand if it is sup- 

 ported on its wings in flight the backbone will be raised. 



