MECHANISM OF RESPIRATION 139 



withstanding the fact that the lungs have an opening (the trachea) 

 by which they communicate with the external air; and, so far as 

 the simple ingress and egress of air is concerned, the question of 

 pulmonary respiration resolves itself into one of pure mechanics. 

 The lungs may be looked upon as a bag (or two bags) in 

 the thoracic cavity. Inspired air does not enter the thoracic 

 cavity, but this bag which is in it. This fact is of the greatest 

 importance. 



Furthermore, the lungs are everywhere in contact with the 

 thoracic wall by their pleural surfaces. They are composed 

 very largely of highly developed elastic tissue, but are perfectly 

 passive themselves. That is to say, they possess no power of 

 expansion except in obedience to extraneous influences. As 

 found in the thorax they possess a contractile power, but only 

 because certain forces have put their elastic tissue on the stretch, 

 and the contraction is a simple effort of the tissue to return to the 

 condition which characterized it before it was subjected to the 

 expanding force. 



Before birth there is no air in the lungs, and this is the only 

 time when the elastic tissue is not on the stretch. The bronchi- 

 oles and air cells are collapsed, but the thorax is contracted and 

 the pulmonary and thoracic walls are in contact by their respec- 

 tive pleural surfaces. When the child is born an inspiration 

 fills the lungs and they are never thereafter devoid of air. They 

 collapse to a certain extent and leave the thoracic wall when the 

 chest is opened, but cannot empty themselves entirely because the 

 walls of the bronchioles collapse before all the air can escape. 

 This collapse of the lungs when the chest wall is opened shows 

 that the lung structure is in a constant state of tension, which 

 tension has always a tendency to empty the lungs, but cannot do 

 so because the thorax can contract only so far, and when its con- 

 traction has reached its limit, for the lung to contract farther 

 would mean a separation of the pulmonary and thoracic walls 

 and the formation of a vacuum between them. The additional 



