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ANIMAL BIOLOGY 



B. Respiratory Mechanism 



In order for the lungs to play their part in respiration, it is 

 evident that the air within them must be periodically renewed, 

 and this rhythmical process of inhalation and exhalation is 

 what one usually refers to as breathing. The complex mechanism 

 involved and its method of operation may be briefly outlined. 



The lungs are elastic sacs suspended in an air-tight cavity, the 

 thorax, which can be enlarged by raising the rirs and lowering 

 the diaphragm, a muscular partition between the thoracic and 



Fig. 119 A. — Diagram to illustrate the mechanism of diaphragm breathing. 

 The lungs of a Mammal are enclosed in a bell-jar. As the rubber membrane 

 below (representing the diaphragm) is pulled down enlarging the cavity, air 

 enters through the tube (trachea) and expands the lungs. (From Conn and 

 Budington, after Tigerstedt.) 



abdominal cavities. The sole entrance to the lungs is through the 

 trachea, and accordingly an atmospheric pressure of approximately 

 fifteen pounds to the square inch is exerted down through the 

 trachea on the inner walls of the lungs and keeps them constantly 

 in close contact with the walls of the thoracic cavity — otherwise 

 there would be a vacuum between the lungs and the thoracic wall. 

 Therefore when the thoracic cavity is enlarged by contraction of 

 the muscles of the ribs and diaphragm, the elastic lungs expand in 

 maintaining contact with its walls — inspiration takes place ; and 

 when the thoracic cavity is decreased, by relaxation of the same 

 muscles, expiration occurs. The lungs play an entirely passive 



