1 68 Life and Health 



chest expands, and there is an inrush of fresh air. This is 

 known as inspiration. 



This is at once followed by the falling of the chest walls 

 and the expulsion of air. This is known as expiration. 



A pause now occurs, and the same breathing movements 

 are repeated. 



254. The Number of Respirations. In a healthy adult 

 the number of respirations should be from 16 to 18 per 

 minute. Exercise increases the number, while rest dimin- 

 ishes it. In standing the rate is more than when lying at 

 rest. Mental excitement quickens the rate. The rate is 

 lowest during sleep. 



In diseases involving the lungs, bronchial tubes, and the 

 pleura, the rate may be alarmingly increased, and the pulse 

 is quickened in proportion. 



The entrance and the exit of air into the respiratory 

 passages are accompanied with peculiar sounds, which are 

 readily heard on placing the ear at the chest wall. 

 ^255. The' Chest as an Air-Tight Chamber. The chest is 

 an air-tight chamber with movable walls and floor. It has 

 bony walls, with ribs connecting in front with the breast- 

 bone and behind with the spine. The spaces between the 

 ribs are occupied by the intercostal muscles, while large 

 muscles clothe the entire chest. The diaphragm serves as 

 a movable floor to the chest. 



In this air chamber are suspended the lungs, the air cells 

 of which communicate with the outside through the bron- 

 chial passages, but have no connection with the chest cavity. 

 The thin space between the lungs and the walls of the 

 chest, called the pleural cavity, is in health a vacuum. 



256. The Mechanism of Normal Respiration. Let us 

 now study the chief points about the mechanism of breathing, 

 or respiration. 



