THE MECHANICS OF RESPIRATION 321 



enter the central tendon, and the lateral sheets are pressed upward by 

 the intraabdominal positive and intrathoracic negative pressures, so that 

 they form a dome-shaped vault, with the liver in the right side and the 

 stomach and the spleen in the left. 



During expiration the lateral edges of the diaphragm are in contact 

 with the parietal pleura of the thoracic cavity, forming what are known 

 as the pleural sinuses. During inspiration the fibers of the diaphragm 

 shorten; this straightens out the arch of the diaphragm and pulls the 

 lateral edges of the diaphragm away from the parietal pleura, thus open- 

 ing up the pleural sinuses, into which the lungs descend. Usually the 

 opening up of the sinuses is accompanied by a slight retraction of the 

 external chest wall, which is known as Litten's diaphragm phenomenon. 

 The descent of the diaphragm may produce a movement of from 10 to 

 15 mm. on each side, which accounts for a rather important fraction of 

 the volume of air exchange by the lungs. The central portion of the 

 diaphragm does not move much in normal .respiration, but in forced 

 respiration its movement may be considerable. 



Because of its attachments to the lower six ribs, the contraction of the 

 .diaphragm tends to pull the margins of the ribs towards the median line, 

 but under normal conditions this movement is opposed by the action of 

 the external intercostals in raising the ribs and expanding the horizontal 

 diameters of the thorax, and by the lower vertebral muscles, which fix 

 the position of the lower ribs. 



The relative part which the diaphragm and the external intercostal 

 muscles play in the widening of the lower part of the thorax is of som& 

 importance from the standpoint of diagnosis. It has generally been held 

 that the contraction of the diaphragm produces a widening of the lower 

 part of the thorax, because in its descent it presses upon the abdominal 

 viscera and so distends the abdomen and pushes out the lower ribs. 

 That this might occur seems not improbable, but Hoover 2 has recently 

 shown by experimental and clinical observations that the flaring in the 

 costal margins seen in normal inspiration depends on other factors. He 

 calls attention to the fact that the contraction of the intercostals raises 

 the ribs and increases the angular divergence of the subcostal borders. 

 This widening of the angle made b'y the costal margins at the tip of the 

 sternum is very pronounced in paralysis of the diaphragm while in 

 paralysis of the intercostal muscles, the costal borders are drawn towards 

 the median line and the subcostal angle is decreased. This shows that 

 the diaphragm must tend to diminish the angle. 



The line of traction of the diaphragm is a straight one joining the cen- 

 tral tendon with the edge of the ribs. When the diaphragm forms a 

 well-defined arch, it exerts its traction at a disadvantage, and the ex- 



