318 THE MAINTENANCE OF THE INDIVIDUAL 



COMPOSITION OF FRESH AIR AND THAT EXPIRED FROM THE LUNGS 



Constituents 



Oxygen 



Carbon dioxide . . . . 

 Nitrogen and other gases 

 Water vapor 



In Outdoor Aik 



20.96 

 .04 

 79.0 

 variable 



In Air Expired 

 FROM THE Lungs 



16.4 



4.1 



79.0 



.5 



As shown in the above table, there is a loss of nearly 5 per cent of 

 oxygen and a corresponding gain in carbon dioxide and water vapor 

 in expired air. 



The lungs are located in a triangular, air-tight sac or thoracic cavity, 

 with the sternum or breastbone in front, the ribs on the side, the 

 immobile vertebral column at the back, and the convex diaphragm 

 below. The ribs, connected to the breastbone in front and the back- 

 bone behind, are united to each neighboring rib by a sheet of intercostal 

 muscles. Furthermore the articulation of the rib with the vertebral 

 column is higher than its connection with the sternum, and the shape 

 is such that when the lungs are empty the "convexity of the curve 

 points slightly downwards." Inspiration results from the contraction 

 of the intercostal and associated muscles which not only pull the 

 ribs toward a horizontal position but also force the sternum ventrally. 

 The diaphragm., which also assists, is a combination of a membrane 

 and muscle and forms a partition between the thoracic and abdominal 

 cavities. The concave surface of the diaphragm is towards the pos- 

 terior, that is, down. Contraction reduces the concavity so that the 

 result is an increase in the capacity of the thoracic cavity. Keeping 

 in mind that the chest cavity is air tight, the lungs elastic, and that 

 the sole entrance of air is from the trachea, it is not difficult to see 

 that when the capacity of the chest cavity is increased by the move- 

 ments described above, the lungs naturally expand and inspiration 

 takes place. Expiration is produced in part by special muscles, the 

 relaxation of the diaphragm and walls of the chest cavity, and the 

 elasticity of the lungs themselves. 



The nervous mechanism that controls this process is found in the 

 respiratory center of the medulla oblongata (see page 351.) Under 

 normal conditions respiration results from the alternate stimulation of 

 two sets of fibers in the vagus nerve leading from the lungs to the 

 respiratory center. The inspiratory fibers are stimulated at each ex- 

 piration by the collapse of the lungs, which results in an increase in 



