Diseases of the Respiratory System 



149 



ventral side of the neck, while diverticula from the large sacs 

 are embedded among the muscles and even penetrate some 

 of the bones. These sacs function chiefly as reservoirs of 

 air. Some aeration of blood takes place in the sacs and they 

 also help to reduce the relative weight of the body. The air 

 sacs are the elastic or 

 bellows-like portion of 

 the respiratory appara- 

 tus. The lungs, on ac- 

 count of their structure 

 and position in the body, 

 are permanently dis- 

 tended. During inspira- 

 tion the air passes 

 through the trachea and 

 lungs into the sacs. 

 Fresh outside air is thus 

 brought into the portion 

 of the lungs where the 

 blood is aerated. During 

 expiration air from the 

 sacs is forced back 

 through the lungs. The 

 current sucks out the air 

 from the blind ending 

 tubes and to some extent 

 supplies comparatively fresh air in expiration. Thus the 

 respiratory apparatus in birds is more efficient than in 

 mammals, where fresh air is never available for the aera- 

 tion of the blood, the entire process being carried on by 

 residual air. 



In addition to aeration of the blood, the respiratory ap- 

 paratus eliminates most of the waste moisture of the body and 

 is, therefore, the temperature regulator. In mammals this 



Pig. 27. — Ventral surface of the right 

 lung of a fowl, injected with wax. 

 (From Gadow, after Stieda.) 



