CAPACITY OF THE LUNGS. 365 



sternum to the skull, are used to pull up the former. The 

 muscles which are thus called into play in labored but not 

 in quiet breathing are called extraordinary muscles of res- 

 piration. 



The Respiratory Sounds. The entry and exit of air 

 are accompanied by 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. 



The Capacity of the Lungs. Since the chest cavity 

 never even approximately collapses, the lungs are never 

 completely emptied of air: the space they have to occupy 

 is larger in inspiration than during expiration but is always 

 considerable, so that after a forced expiration they still con- 

 tain a large amount of air which can only be expelled from 

 them by opening the pleural cavities; then they entirely 

 collapse, just as the bag in Fig. 107 would if the bottle in- 

 closing it were broken. The capacity of the chest, and 

 therefore of the lungs, varies much in different individuals, 

 but in a man ot medium height there remains in the lungs 

 after the most violent possible expiration, about 1640 cub. 

 cent. (100 cub. inches) of air, called the residual air. 

 After an ordinary expiration there will be in addition to 

 this about as much more supplemental air; the residual and 

 supplemental together forming the stationary air, which 

 remains in the chest during quiet breathing. In an ordi- 

 nary inspiration 500 cub. cent, (30 cub. inches) of tidal air 

 are taken in, and about the same amount is expelled in nat- 

 ural expiration. By a forced inspiration about 1600 cub. 

 cent. (98 cub. inches) of complemental air can be added to 

 the tidal air. After a forced inspiration therefore the chest 

 will contain 1640 -f 1640 + 500 -j- 1600 = 5380 cubic centi- 

 meters (328 cubic inches) of air. The amount which can 

 be taken in by the most violent possible inspiration after 

 the strongest possible expiration, that is, the supplemental, 



