RESPIRATION. 345 



the chest by means of which air is drawn into or expelled from the 

 lungs. These two acts have received the respective names: inspira- 

 tion and expiration. As is known, the whole external surfaces of 

 the lungs are in direct contact in an air-tight manner with the inner 

 wall of the thorax, so that the lungs must be distended with every 

 dilatation of the thoracic wall as well as be diminished in volume by 

 every contraction of the same wall. The movements of the lungs 

 are, therefore, for the most part, passive, being dependent upon the 

 movements of the thoracic wall. This close approximation of lung 

 to thoracic wall is dependent upon a state of elastic tension main- 

 tained within the lung, due to pressure exerted by the presence in 

 the lung of residual air. 



Fig. 116. Diagrammatic Representation of the Action of the 

 Diaphragm. ( BECLABD. ) 



If a represents a plane extending in expiration from the sternum to the 

 vertebra, and D the position of the diaphragm in inspiration, the plane a will 

 move to A, while the diaphragm will descend to d. 



From these data it becomes evident that all that is necessary 

 for the production of inspiration is such a movement of the walls 

 or the diaphragm, or the movement of the two synchronously, that 

 the capacity of the interior should be increased. By reason of this 

 increase there would be produced a temporary vacuum in the newly 

 acquired space, or at least a great diminution of pressure within the 

 lungs, so that atmospheric pressure upon the outside is greater than 

 that within. Consequently there will be generated a current of air 

 proceeding from the outside air through the larynx and trachea into 

 the lungs, for the purpose of equalizing the pressure upon the inside 

 and outside of the chest. The moment this point is reached there 

 is cessation of the current. This incoming of the air constitutes the 

 first of the two acts of respiration, namely : inspiration. 



For the expulsion of the air that is no longer fit for oxidation 



