EXPIRATION. 245 



rium. In ordinary expiration, on the contrary, but little 

 if any muscular effort is required. 



As soon as the muscles which have raised the ribs and 

 sternum relax, these bones return to their natural uncon- 

 strained position; and the elastic abdominal wall presses 

 the abdominal viscera against the under side of the dia- 

 phragm and pushes that organ up again, as soon as its 

 muscular fibres cease contracting. In this way the chest 

 cavity is restored to its original capacity, and the air is 

 sent out of the lungs rather by the elasticity of the parts 

 which were stretched in inspiration, than by any special 

 expiratory muscles. 



When, however, an expiration is violent (when, for ex- 

 ample, we try to empty our lungs of air as completely as 

 possible, or during a fit of coughing) special expiratory 

 muscles, which pull down the ribs and press up the dia- 

 phragm, are called into action. 



The Respiratory Sounds. The entry and exit of air are 

 accompanied by the respiratory sounds or murmurs, which 

 can be heard on applying the ear to the chest wall. The 

 character of these sounds is different and characteristic 

 over the trachea, the larger bronchial tubes, and portions 

 of lung from which large bronchial tubes are absent. They 

 are variously modified in pulmonary affections, and hence 

 the value of auscultation of the lungs in assisting the phy- 

 sician to form a diagnosis. 



Hygienic Remarks. Since the diaphragm when it con- 

 How does expiration differ from it in this respect? 

 Explain how the chest is brought hack to its resting position after 

 an inspiration? 



Give examples of violent expiration? How does it differ from 

 an ordinary expiration in the forces at work for its production? 



What are the respiratory sounds? Where do their characters 

 differ? Why do physicians study them in lung diseases? 



