MECHANISM OF RESPIRATION 139 



MECHANISM OF RESPIRATION. 



Respiration implies the more or less regular entrance and 

 exit of air to and from the lungs. The entrance is inspira- 

 tion; the exit expiration. Now, the thorax is a closed cav- 

 ity, notwithstanding 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 per- 

 fectly passive themselves. That is to say, they possess no 

 power of expansion except in obedience to extraneous in- 

 fluences. 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 t]je 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 

 bronchioles and air cells are collapsed, but the thorax is con- 

 tracted and the pulmonary and thoracic walls are in contact 

 by their respective 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 col- 

 lapse 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 be- 



