BREATHING 283 



One lung serves to keep up respiration for the time and 

 after the wound is closed the air between the chest 

 wall and the contracted lung is gradually absorbed. 

 In proportion as it is removed the lung approaches its 

 full size and at length the layers of the pleura are once 

 more in approximate contact. A lung may be reduced 

 in volume and in usefulness by the accumulation of 

 liquid as well as air outside it and it is sometimes neces- 

 sary to withdraw such fluid through a puncture between 

 the ribs. 



The Breathing Movements. Generally speaking, any 

 movement which enlarges the chest will enlarge the 

 lungs. The enlargement will affect chiefly the elastic 

 terminal sacs which are much more susceptible to stretch- 

 ing and recovery than the blood-vessels or the bronchial 

 tubes between these sacs and the bronchi. When the 

 sacs are made larger air will press into them as into a 

 widening bellows. It is the atmospheric pressure which 

 drives it and not a muscular application like that of 

 swallowing. Any movement which makes space for 

 the reception of air is reckoned a movement of inspira- 

 tion. From what has been said of the elastic tension 

 of the lungs it should be evident that these organs resist 

 inspiration and assist in expiration. The one is made 

 more difficult than if the lungs were indifferent to it, 

 while the other is facilitated. The condition may be 

 likened to that illustrated by a screen door which has a 

 spring: it is harder to open than if it had none, but it 

 closes itself. 



Elevation of the ribs is an important factor in inspira- 

 tion. It is not necessary to analyze the action of the 

 muscles by which this is brought about, but the effect 

 upon the dimensions of the thorax must be made plain. 

 There are twelve pairs of ribs. Each rib is hinged upon 

 one side of a vertebra. It connects with the vertebra 

 at two points and, this being the case, its movement is 

 limited to rotation around an axis passing through these 

 two points. The direction of the axes is somewhat 



