436 MANUAL OF ZOOLOGY. 



Respiration is effected in Birds more completely and actively 

 than in any other class of the Vertebrata, and as the result of 

 this, their average temperature is also higher. This extensive 

 development of the respiratory process is conditioned by the 

 fact that, in addition to true lungs, air is admitted into a greater 

 or less number of the bones, and into a number of cavities 

 the so-called air-receptacles which are distributed through vari- 

 ous parts of the body. By this extensive penetration of air into 

 various parts of the body, the aeration of the blood is effected, 

 not only in the lungs, but also over a greater or less extent of 

 the systemic circulation as well ; and hence in Birds this pro- 

 cess attains its highest perfection. The cavities of the thorax 



and abdomen are not separated 

 from one another by a complete 

 partition, the diaphragm being 

 only present in a rudimentary 

 form. The lungs are two in num- 

 ber, of a bright-red colour, and 

 spongy texture. They are confined 

 to the back of the thorax, extend- 

 ing along each side of the spine, 

 from the second dorsal vertebra 

 to the kidney. They differ from 

 the lungs of the Mammals in 

 not being freely suspended in 

 a pleural membrane. The pleura, 

 on the other hand, is reflected 

 only over the anterior surface of 

 the lungs. The bronchi, or 

 primary divisions of the wind- 

 pipe (fig. 172), dimmish in size 

 as they pass through the lung, 

 by giving off branches, which, in 

 turn, give off the true air-vesicles 

 of the lung. When the bronchial 

 tubes reach the surface of the 

 lung, they open, by a series of dis- 

 tinct apertures, into a series of 

 " air-sacs." These are a series of 

 membranous sacs formed by the 

 continuation of the lining mem- 

 brane of the bronchi, and sup- 

 ported by reflections of the serous membrane of the thoracico- 

 abdominal cavity. In those aquatic birds which, like the 

 Penguin, do not enjoy the power of flight, the air-cells are 



Fig. 172. Lung of Goose (after Owen). 

 a Main bronchus dividing into secon- 

 dary branches as it enters the lung, 

 these giving off smaller branches, the 

 openings of which are seen on the 

 back of the bronchial tubes ; b b Bris- 

 tles passed from the bronchi through 

 the apertures on the surface of the 

 lung by which the bronchi communi- 

 cate with the air-receptacles. 



