354 RESPIRATION [OH. XXTI. 



The Respiratory Mechanism. 



Kespiration consists of the alternate expansion and contraction 

 of the thorax, by means of which air is drawn into or expelled 

 from the lungs. These acts are called Inspiration and Expiration 

 respectively. 



For the inspiration of air into the lungs a movement of the side- 

 walls and floor of the chest takes place, so that the capacity of the 

 interior is enlarged. By such increase of capacity there will be a 

 diminution of the pressure of the air in the lungs, and a fresh 

 quantity will enter through the larynx and trachea to equalise the 

 pressure on the inside and outside of the chest. 



For the expiration of air, the opposite movement diminishes the 

 capacity of the chest; the pressure in the interior will be thus 

 increased, and air will be expelled, until the pressure within and 

 without the chest are again equal. In both cases the air passes 

 through the trachea, there being no other communication with the 

 exterior of the body ; and the lung remains, under all circumstances, 

 closely in contact with the walls and floor of the chest. The move- 

 ments of the lung are therefore passive, not active, and depend on 

 the changes of shape of the closed cavity in which they are contained. 

 A perforation of the chest-wall would mean that the lung on that 

 side would no longer be of use ; a similar injury on the other side 

 (double pneumothorax) would cause death. If the two layers of the 

 pleura were adherent, those portions of the lung would be expanded 

 most where the movements of the chest are greatest. The existence 

 of the two layers prevents this, and thus the lung is equally expanded 

 throughout. 



Inspiration. This is a muscular act ; the effect of the action of 

 the inspiratory muscles is an increase in the size of the chest-cavity 

 in the vertical, the lateral, and antero-posterior diameters. 



The vertical diameter of the chest is increased by the contraction 

 and consequent descent of the diaphragm ; at rest, the diaphragm is 

 dome-shaped with the convexity upwards ; the central tendon forms 

 a slight depression in the middle of this dome. On contraction the 

 muscular fibres shorten, and so the convexity of the double dome is 

 lessened. The central tendon is drawn down a certain distance, but 

 the chief movement is at the sides. For the effective action of this 

 muscle, its attachment to the lower ribs is kept fixed by the con- 

 traction of the quadratus lumborum. The diaphragm is supplied by 

 the phrenic nerves. 



The increase in the lateral and antero-posterior diameters of the 

 chest is effected by the raising of the ribs, the upper ones being fixed 



