RESPIRATION 



35 



but the ultimate air cells of the lungs, these organs 

 are but variously modified tubes through which the air 

 is drawn, heated and moistened. It is in the air cells that 

 the thin Avails of the pulmonary capillaries bring the 

 blood close enough to the inspired air for the exchange 

 to take place. 



Respiratory Forces. For normal respiration the re- 

 spiratory forces are the diaphragm and certain muscles 



Fig. 8. The position of the lungs in the thorax. (T. Wingate Todd.) 

 (Pearce-Macleod, Fundamentals of Human Physiology.) 



which move the ribs. Essentially the lungs are a pair 

 of elastic bags, communicating with atmospheric air 

 through the breathing tubes, enclosed in a movable bony 

 framework which is covered by soft tissues and lined, 

 for each lung, by a frictionless membrane completely 

 air-tight. If the framework increases in size, the lungs 

 must either follow it or leave a vacuum between lung 



