THE RESPIRATORY SYSTEM. 247 



posticus (posterior) superior. Since the ribs slant downwards and 

 forwards from their vertebral articulation, this upward movement 

 carries forward the sternum and increases the antero-posterior diameter 

 of the chest. At the same time the ribs rotate slightly round the axis 

 represented by a line drawn from their vertebral to their costal attach- 

 ments, and their lower borders, which in the expiratory position are 

 inverted, become everted. The costo-chondral angle is also opened out. 

 By these means the transverse diameter of the chest is enlarged. 



The lower ribs are raised partly by the external intercostal muscles, 

 partly also by the contraction of the diaphragm, and partly by the 

 interchondral portion of the internal intercostal muscles. 



Thus the muscles concerned in quiet inspiration are the diaphragm, 

 the external intercostal muscles, the interchondral portion of the internal 

 intercostal muscles, and the serratus posticus (posterior) superior. 

 The entrance of air into the lungs is also assisted by widening of the 

 glottis, and, if the breathing is at all laboured, by dilatation of the alae 

 nasi. In forced inspiration, other muscles such as the trapezius, pectoral 

 muscles, sterno-mastoid, and rhomboids are called into play. 



During quiet expiration, the chest returns to its former shape and 

 size, mainly on account of the elasticity of the chest wall and lungs, 

 and of the abdominal wall and abdominal contents ; the downward 

 movement of the ribs during expiration is also assisted by the con- 

 traction of the costal part of the internal intercostal muscles, which 

 pass downwards and backwards from each rib to the one immediately 

 below it. In forced expiration the accessory muscles employed are 

 mainly those of the abdominal wall. 



Quiet respiration in men is carried out principally by the movements 

 of the diaphragm. In women the larger part is played by the move- 

 ments of the upper ribs, chiefly on account of wearing of tight clothing, 

 which interferes with the movement of the diaphragm and lower ribs. 



THE EFFECT OF THE RESPIRATORY MOVEMENTS 

 ON THE LUNGS. 



The passage of air into and out of the lungs during respiration is 

 brought about by purely mechanical causes, which can be roughly 

 illustrated by the aid of an artificial model. A thin- walled rubber bag 

 is placed in a glass vessel closed by a cork ; the bag is attached to a 

 glass tube, which passes through the cork and is open at the top. 

 The bottle is connected by another tube with a mercury manometer 

 and by a third tube with a suction pump, by means of which air can 

 be sucked out of it. 



