302 



EXTERNAL RESPIRATION 



sides. When the sides are pulled outwards the box increases in 

 capacity and the air is sucked into the bag to keep the pressure 

 constant. When, however, the force which drew the sides out- 

 wards is released, the box and bag re- 

 sume their former volumes, and air is 

 expelled. In short the lungs are a form 

 of suction pump or bellows (Fig. 68). 



Structure of Mechanism. 



While the foregoing account of the 

 principle underlying the respiratory 

 mechanism may be taken as substan- 

 tially correct, it is apt to convey a 

 wrong impression of the details of the 

 mechanism. 



(a) The lungs are not simple elastic 

 bags but are composed of thousands of 

 little distensible air sacs a sponge- 

 work. 



(b) These complex bags are suspended 

 in and almost fill the thoracic cavity. 

 Each organ is enclosed in a membranous 

 sac the pleura, which bends back from 

 the bronchi and lines the entire internal 

 surface of the chest wall. That is, the 

 P^ura consists of two layers an outer, 



,'s and they expand. par i e tal or chest wall layer, and an 

 inner, visceral or lung layer. These surfaces are kept moist 

 with lymph. It is important to note that as long as the chest wall 

 is kept intact the pleural cavity is only a cavity in name. The 

 layers of the pleura are always normally in close contact with one 

 another and with the underlying and overlying surfaces. In 

 other words, the chest wall, the two layers of the pleura and the 

 outer surface of the lungs move almost as one structure. The 

 elasticity of the lungs has been determined as about 30 mm. Ilg. 

 If this inwards pull of the pulmonary tissue be subtracted from the 

 atmospheric pressure (760 mm.) in the lung, the resulting figure 

 (730 mm.) represents the force tending to keep the lungs expanded. 

 If, now, communication be established between the outer air and 

 the intra-pleural cavity, there will be a pressure of 760 mm. 

 tending to cause the lungs to collapse. As these outwards and 

 inwards pressures (760 as against 730 mm.) do not balance, one 



