THE BREATHING SYSTEM 



481 



muscles contract in order to help push the 

 diaphragm upward. These muscles also 

 play an important part in coughing and 

 sneezing. Preceding these actions, the air 

 passages are voluntarily closed and the 

 muscles contract to build up a pressure 

 within the luno;s. This is followed by a sud- 

 den opening of the passages, resuitmg m 

 an explosive discharge of air which is de- 

 signed to dislodge any foreign matter that 

 may have gotten into the air passages. 



Accidental puncturing of the thoracic 

 cavity brings about the collapse of the lung 

 because when air within the pleural cavity 



again after about two years as the air is 

 slowly absorbed into the tissues and even- 

 tually carried away in the blood. 



In some diseases, particularly polio- 

 myelitis, the motor nerves to the breathing 

 mechanism fail to function, resulting in 

 death from suffocation if some artificial 

 breathing mechanism is not brought into 

 immediate action. This apparatus, commonly 

 called an "iron hmg," is merely a device 

 that creates a partial vacuum over the chest 

 region, thus replacing the non-functioning 



Fig. 18-4. Lungs can be deflated by simply forcing air 

 into the pleural space as shown here. 



reaches the same pressure as the outside 

 air, the lung contracts, as pointed out 

 above. Such a lung is non-functional and if 

 both lungs collapse at the same time, suffo- 

 cation follows. Fortunately, there is a parti- 

 tion between the lungs so that one can be 

 deflated while the other functions normally. 

 It even becomes necessary sometimes to 

 collapse one or the other lung artificially 

 in such diseases as tuberculosis. This is 

 easily accomplished by simply forcing a 

 hollow needle between the ribs into the 

 pleural space and allowing sterile air to 

 enter (Fig. 18-4). When the needle is with- 

 drawn the lung will gradually fill the space 



Fig. 18-5. In such diseases as poliomyelitis the motor 

 nerves to the breathing mechanism fail. The iron lung 

 is a device that substitutes for the action of the 

 breathing muscles. A partial vacuum is produced in 

 a rhythmic manner over the entire body, but its spe- 

 cific effect is on increasing and decreasing the thoracic 

 cavity, simulating normal breathing. 



muscles (Fig. 18-5). Its use must be con- 

 tinued as long as the muscles remain in- 

 active. Frequently the nerves regenerate, in 

 part at least, so that gradually they are able 

 to carry impulses to the breathing muscles 

 and normal breathing is restored. 



LUNG CAPACITY 



It is a familiar fact that breathing is slow 

 and shallow when one is at rest, either 

 sitting or lying down but rapidly becomes 

 faster and deeper with exertion such as 

 running. It is also well known that when 

 resting one can take in a great deal more 



