134 NUTRITION OF FARM ANIMALS 



The walls of the trachea and bronchi consist of cartilaginous 

 rings which prevent them from collapsing. The alveoli and 

 bronchioles are surrounded and bound together by connective 

 tissue consisting largely of elastic fibers so that the minute air 

 cavities of the lungs are extensible and their walls elastic. In 

 this connective tissue are found the larger branches of the 

 pulmonary artery and pulmonary vein, connected by a net- 

 work of capillaries which are spread out over the inside of the 

 alveoli in direct contact with their lining membrane. Each 

 lung is enclosed in a double-walled sack, the pleura, one wall 

 of which covers the lungs and the other the chest walls and 

 diaphragm, the narrow cavity between the two being filled 

 with a liquid. 



190. Mechanics of breathing. In breathing, the lungs 

 themselves play an essentially passive role, the movement of 

 air into and out of them being effected by changes in the capac- 

 ity of the chest brought about by the movements of the dia- 

 phragm and ribs. 



Since the diaphragm is convex toward the chest its contrac- 

 tion tends to pull the apex of the dome toward the abdomen, 

 'thus increasing the volume of the chest cavity and by pressure 

 on the digestive organs distending the abdominal walls. When 

 the diaphragm relaxes again the volume of the chest is reduced 

 and the abdominal walls return to their former position. This 

 type of breathing is what is called abdominal breathing. 



The ribs pass obliquely around the chest from the spine to 

 the breast bone (sternum). By means of the intercostal 

 muscles, located between .them, the ribs can be elevated, 

 turning on their attachments to the spine and sternum, thus 

 increasing the diameter of the chest both from side to side 

 and from front to back and so increasing the capacity of the 

 chest cavity. This type of breathing is called costal, or rib, 

 breathing. 



The two types of breathing are ordinarily combined. By 

 their joint action the size of the closed pleural cavity contain- 

 ing the lungs is increased and the atmospheric pressure forces 

 more air into the extensible alveoli of the lungs, so that the 

 latter expand along with the chest cavity, the whole constitut- 

 ing the act of inspiration, or breathing in. When the dia- 

 phragm and the intercostal muscles relax, the elasticity of the 



