RESPIRATION 



111 



amount of this depression varies between 1 cm. in ordinary respiration 

 to 2 or 3 cm. in deep inspiration. 



The expiratory muscles are far less important than the inspiratory 

 because the process of expiration is largely a passive recoil of tissues 

 twisted or stretched in inspiration. The lungs also, as we have seen, 

 are highly elastic and tend ever to contract to a size much smaller than 

 that when they are full. The muscles of the abdominal wall also aid 



FIG. 65 



Apparatus to show the pneumatic relations of the respiration and of the circulation: A, rep- 

 resents the thorax; B, the diaphragm; C, the glottis; D is a tube leading to the manometer, 

 7, indicating the intrathoracic pressure (while E runs to another manometer (not shown) indicating 

 the intrapulmonary pressure); G is a reservoir (veins), and H a receiver, connected in part by 

 the thin loose tube F; at V and V' are valves; K represents the muscles which lower the dia- 

 phragm in inspiration. When this occurs the heart is distended and the suction helps to draw the 

 blood from G toward the heart F, the valve V' preventing suction also on H. The same suction 

 draws apart the suspended rubber bags (lungs) , and to fill this increased space air falls in through 

 the opening C (glottis). When the diaphragm ascends in expiration the reverse processes occur. 

 (Hering.) (See Experiment 30 in the Appendix.) 



expiration by contracting to a slight extent, pressing the abdominal con- 

 tents against the diaphragm. 



The internal intercostal muscles, put on stretch by inspiration, tend to 

 contract in expiration, although in ordinary breathing they are used 

 apparently mostly to complete the thoracic wall between the ribs. In 

 dyspnea (difficult breathing), the abdominal muscles, especially the 



