elasticity of the inflated lung tends to pull 

 the lung away from the thoracic wall, gener- 

 ating a partial vacuum in the pleural cavity. 

 Thus to collapse a lung while it is still in 

 the body — and this is sometimes necessary in 

 severe tuberculosis — the surgeon merely 

 punctures the thoracic wall. This operation 

 permits air to flow directly into the pleural 

 cavity, destroying the partial vacuum and 

 allowing the lung to deflate itself as a result 

 of its own elasticity. 



Breathing Movements. Adequate aeration 

 of the blood in the lungs depends upon an 

 efficient ventilation of the alveolar air spaces; 

 and in most animals the breathing move- 

 ments go on continuously. Each inspira- 

 tion is quickly followed by an expiration, 

 and in man, these respiratory movements 

 recur about 18 times per minute from birth 

 to death. 



Inspiration involves a coordinated contrac- 

 tion of a number of muscles; and expiration 

 follows when the same muscles relax. The 

 muscles most important in breathing are the 

 diaphragm and the intercostal muscles. The 

 action of the rib muscles is to broaden and 

 deepen the thorax, increasing its cubic ca- 

 pacity (Figs. 19-5 and 19-6). The intercostal 

 muscles raise the ribs and swing them later- 



Respiration - 361 



ally, more or less like the handle of a bucket 

 (Fig. 19-5); and simultaneously the dome- 

 shaped diaphragm contracts, lowering the 

 floor of the thorax (Fig. 19-6). Such an en- 

 largement of the thorax increases the partial 

 vacuum in the pleural cavity, and this suc- 

 tion is transmitted through the lungs to the 

 outside air. Thus, as soon as the intrapul- 

 monary pressure falls below that of the out- 

 side atmosphere, air passes into the lungs 

 via the trachea and other passages. 



Expiration usually involves a simple re- 

 versal of these movements. The relaxing 

 intercostal muscles allow the ribs to spring 

 back to their former position; the diaphragm 

 encroaches upon the thorax from below; and 

 both these processes allow the lungs to expel 

 the air that previously was inhaled. 



Extra deep, or forced, breathing requires 

 the cooperation of the abdominal muscles. 

 At the apex of the fullest inspiration the dia- 

 phragm is greatly depressed. Thus consider- 

 able pressure would be exerted upon the 

 abdominal organs, were it not for the fact 

 that simultaneously there is a relaxation of 

 the muscles of the abdominal wall. Con- 

 versely, at the extremity of a forced expira- 

 tion, the muscular abdominal wall clamps 

 down, forcing the viscera to push the dia- 



Fig. 19-6. Normal chest, at end of expiration (left); and at end of inspiration 

 {right). Note that during inspiration (1) the ribs and breastbone are elevated, 

 and (2) the diaphragm and heart are depressed. (Courtesy, Roentgenology Staff, 

 Billings Hospital. From Gerard, The Body Functions. Permission of John Wiley and 

 Sons, Inc.) 



